Final answer:
The circuit breaker to always check prior to APU start is the APU circuit breaker.
Step-by-step explanation:
The circuit breaker that you always check prior to APU start is the APU circuit breaker.
APU stands for Auxiliary Power Unit, which is a device used to provide power to various electrical systems in an aircraft while it is on the ground. The APU circuit breaker is checked before starting the APU to ensure that it is not tripped or open, which could indicate a fault or malfunction.
Checking the APU circuit breaker is important for safety and to ensure that the APU starts and operates correctly.