What safety feature prevents the APU from starting if the temperature is too low?

The correct answer is a temperature sensor. This safety feature is crucial for the APU's operation because extremely low temperatures can adversely affect the fuel and lubrication systems. If the ambient conditions are too cold, the engine components may not function properly, leading to potential failures or damage. The temperature sensor monitors the ambient temperature and ensures that the APU will not attempt to start unless temperatures are within the safe operational range. This precaution is vital for ensuring the longevity and reliability of the APU, safeguarding both the equipment and personnel involved.

The other options do not serve the same purpose: a fuel pressure sensor monitors the fuel supply to ensure that the system has enough pressure for operation, an ambient light sensor is unrelated to APU functioning and typically measures light levels for cockpit or environmental controls, and a circuit breaker provides electrical protection but does not directly monitor environmental conditions impacting the APU's start-up sequence.