  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
$\lambda1.0693\,\mu \rm{m}$ line at $11.9 \times 10^3$~km s$^{-1}$, distinct
from the prominent Mg II $\lambda1.0927\,\mu \rm{m}$ feature, which weakens
toward maximum light. SN 2015bp also displays a clear C II $\lambda6580$A notch
early ($t = -10.9$ days) at $13.2 \times 10^3$~km s$^{-1}$, consistent with our
NIR carbon detection. At $M_B = -$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
$\lambda1.0830\,\mu\rm{m}$ 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
\lesssim-$4 days). We find that four display C II $\lambda$6580A, while two
others show tentative detections, in line with the SN Ia population as a whole.
We conclude that at least $\sim$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.
