Final answer:
In a fire size-up during a rescue operation, determining the cause of the fire is the least important question compared to ensuring the safety of civilians, assessing potential structural collapse, and evaluating the size of the building.
Step-by-step explanation:
While conducting a fire size-up after a storm has damaged a structure, and considering the safest course of action, the least important question among the options provided would be C) What caused the fire? During an immediate crisis situation, the priority is to ensure the safety of civilians and first responders, which includes determining if anyone is trapped inside, assessing the structural integrity of the building to prevent injury from collapse, and estimating the size of the building to understand the scope of the fire and resources needed. The cause of the fire, while important in the investigation phase, does not contribute to the immediate safety and tactical considerations of the rescue operation.