The correct answer is A) controller circuit breaker. If the motor does not start when the start button is pressed, the first thing to check is the controller circuit breaker, as it is responsible for providing power to the motor. The circuit breaker may have tripped due to an electrical fault or overload, preventing the motor from starting. Checking and resetting the circuit breaker is the logical first step in troubleshooting the issue. The other options are not the most likely cause of the problem. The controller contactor operating coil (B) may have an issue, but that would typically be checked after the circuit breaker. The pump discharge relief valve setting (C) and the suction strainer condition (D) are more likely to affect the motor's performance once it is running, rather than preventing it from starting in the first place.