Why must airplanes and fuel tankers be grounded by a wire during fuel transfer?

During fuel transfer, grounding airplanes and fuel tankers with a wire is essential primarily due to the buildup of static electricity. When fuel is transferred, especially at high velocities, the movement of fuel can generate static charges through friction. If these charges accumulate and discharge in an uncontrolled manner, they can create sparks, which pose a significant fire and explosion risk in environments with flammable materials such as aircraft and fuel tankers.

Grounding provides a safe pathway for static electricity to dissipate into the ground, preventing any possibility of a discharge igniting the flammable vapor present in the vicinity. This practice helps ensure the safety of the operation and protects both personnel and equipment from potential hazards associated with static electricity.

Other options relate to different phenomena but do not specifically address the electrical hazards associated with static buildup during fuel transfer. Balancing pressure, avoiding chemical reactions, and preventing overheating are important considerations in various contexts, but they do not directly pertain to the primary reason for grounding in this scenario.