I will agree with the (b) answer.
To answer WattHead's question. As I understand the NEC, there are two derating applications for a conductor. The first is for temperature. Lets assume you have a 75 deg. THHN #6 conductor that passes through a higher than normal temperature area, say 60 Degree C ambient (140F), the typical attic temperature in the summer time. You have to derate the ampacity from the 75 amps by using the multiplier at bottom of Table 310-16. That is 75 X 0.71=53.25Amps. Secondly, lets say you have 4 of those conductors in a conduit. According to Table 310-15(b), the adjustment factor is 80%. That is 53.25 X .8= 42.6 Amps. The termination temperature is still a 60 degree termination.
If you start with a different conductor insulation, such as TW or UF, then the beginning ampacity is at 55 amps for the same #6 conductor. Therefore, if we apply the same multipliers, the final conductor ampacity will be: 55 X .71 X .8= 31.24 amps.
That's how I understand it.
