What safety measure should be taken when refueling near the APU?

When refueling near the Auxiliary Power Unit (APU) of a C-130J, it is essential to shut down the APU as a critical safety measure. The APU is a source of ignition and generates heat, which can pose a significant fire hazard in the presence of fuel vapors. By shutting down the APU, you eliminate the potential for an ignition source that could lead to a fire or explosion during the refueling process.

Ensuring that the APU is off helps create a safer environment for ground crew personnel and reduces the risk of accidents or incidents related to fuel handling. This precaution is a standard procedure in aviation safety to mitigate risks during operations that involve flammable materials, like aviation fuel.

The other options do not align with safety protocols related to refueling. Keeping the APU running would directly increase the risk of a dangerous situation due to the presence of flammable fumes. Using mobile devices with caution may be a good practice in general, but it does not address the immediate danger presented by the APU during refueling. Wearing casual clothing does not contribute to safety and could even pose a risk if proper personal protective equipment isn't worn in a hazardous environment.