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Research by D.J.S. is supported by NSF grants AST-1821967, AST-1821987, AST-1813708, AST-1813466, and AST-1908972, as well as by the Heising-Simons Foundation under grant #20201864. The CSP-II has been supported by National Science Foundation (NSF) grants AST-1008343, AST-1613426, AST-1613455, and AST-1613472, as well as by the Danish Agency for Science and Technology and Innovation through a Sapere Aude Level 2 grant. E.Y.H. and J.L. also acknowledge the support of the Florida Space Grant Consortium. This work was partially performed at the Aspen Center for Physics, which is supported by NSF grant PHY-1607611. Research by S.V. is supported by NSF grants AST1813176 and AST-2008108. L.G. was funded by the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 839090. This work has been partially supported by the Spanish grant PGC2018-095317-B-C21 within the European Funds for Regional Development (FEDER). M.S. is supported by generous grants from Villum FONDEN (13261, 28021) and by a project grant (8021-00170B) from the Independent Research Fund Denmark. A.V.F. is grateful for financial assistance from the TABASGO Foundation, the Christopher R. Redlich Fund, and the Miller Institute for Basic Research in Science (U.C. Berkeley). Based on observations obtained at the international Gemini Observatory (GN-2015A-Q-8, GS-2015A-Q-5), a program of NSF's NOIRLab, which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. on behalf of the Gemini Observatory partnership: the National Science Foundation (United States), National Research Council (Canada), Agencia Nacional de Investigacion y Desarrollo (Chile), Ministerio de Ciencia, Tecnologia e Innovacion (Argentina), Ministerio da Ciencia, Tecnologia, Inovacoes e Comunicacoes (Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea). This paper includes data gathered with the Nordic Optical Telescope (PI Stritzinger) at the Observatorio del Roque de los Muchachos, La Palma, Spain. This work is based in part on observations from the Deep Imaging Multi-Object Spectrograph at the Keck II telescope. We are grateful to the staff at the Keck Observatory for their assistance, and we extend special thanks to those of Hawaiian ancestry on whose sacred mountain we are privileged to be guests. The W. M. Keck Observatory is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA; it was made possible by the generous financial support of the W. M. Keck Foundation. We thank S. Bradley Cenko for assistance with the Keck spectral reductions, as well as Patrick Kelly, WeiKang Zheng, and John Mauerhan for their assistance with the observations. D.J.S. is a visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract 80HQTR19D0030 with the National Aeronautics and Space Administration. Based on data products from observations made with ESO Telescopes at the La Silla Paranal Observatory under programmes 188.D-3003 and 191.D-0935: PESSTO (the Public ESO Spectroscopic Survey for Transient Objects). ; Unburned carbon is potentially a powerful probe of Type Ia supernova (SN) explosion mechanisms. We present comprehensive optical and near-infrared (NIR) data on the "transitional" Type Ia SN 2015bp. An early NIR spectrum (t=−9.9 days with respect to B-band maximum) displays a striking C I λ1.0693μm line at 11.9×103~km s−1, distinct from the prominent Mg II λ1.0927μm feature, which weakens toward maximum light. SN 2015bp also displays a clear C II λ6580A notch early (t=−10.9 days) at 13.2×103~km s−1, consistent with our NIR carbon detection. At MB=−18.46, SN 2015bp is less luminous than a normal SN Ia and, along with iPTF13ebh, is the second member of the transitional subclass to display prominent early-time NIR carbon absorption. We find it unlikely that the C I feature is misidentified He I λ1.0830μm because this feature grows weaker toward maximum light, while the helium line produced in some double-detonation models grows stronger at these times. Intrigued by these strong NIR carbon detections, but lacking NIR data for other SNe Ia, we investigated the incidence of optical carbon in the sample of nine transitional SNe Ia with early-time data (t≲−4 days). We find that four display C II λ6580A, while two others show tentative detections, in line with the SN Ia population as a whole. We conclude that at least ∼50% of transitional SNe Ia in our sample do not come from sub-Chandrasekhar mass explosions due to the clear presence of carbon in their NIR and optical spectra. ; National Science Foundation (NSF) AST-1821967 AST-1821987 AST-1813708 AST-1813466 AST-1908972 PHY-1607611 AST1813176 AST-2008108 ; Heising-Simons Foundation 20201864 ; National Science Foundation (NSF) AST-1008343 AST-1613426 AST-1613455 AST-1613472 ; Danish Agency for Science and Technology and Innovation ; Florida Space Grant Consortium ; European Commission 839090 ; Spanish grant within the European Funds for Regional Development (FEDER) PGC2018-095317-B-C21 ; Villum Foundation 13261 28021 ; Independent Research Fund Denmark 8021-00170B ; TABASGO Foundation ; Christopher R. Redlich Fund ; Miller Institute for Basic Research in Science (U.C. Berkeley) ; International Gemini Observatory GN-2015A-Q-8 GS-2015A-Q-5 ; W.M. Keck Foundation ; University of Hawaii 80HQTR19D0030 ; National Aeronautics & Space Administration (NASA) ; ESO Telescopes at the La Silla Paranal Observatory 188.D-3003 191.D-0935: PESSTO