Final answer:
The minimum size THHN conductor permitted to terminate on a 50A circuit breaker or fuse with 75°C conductor sizing is 6 AWG.
Step-by-step explanation:
According to the information provided, we need to determine the minimum size THHN conductor that's permitted to terminate on a 50A circuit breaker or fuse if the circuit breaker and equipment terminals are listed for 75°C conductor sizing.
Based on the National Electrical Code (NEC), the conductor size can be determined using Table 310.15(B)(16) which provides the ampacity for different sizes of conductors at different temperatures. For a 50A circuit and 75°C conductor sizing, we find that the minimum size THHN conductor permitted is 6 AWG.
Therefore, the correct answer is c. 6 AWG.