The correct answer is A) liberate air pockets and reduce waterside oxidation. Water boxes on main condensers are vented to liberate air pockets that can form and accumulate in the condenser. This venting helps prevent air from being drawn into the system, which could lead to reduced heat transfer and increased waterside oxidation. Removing air pockets maintains efficient heat transfer within the condenser. The other answer choices are incorrect because: B) preventing excessive pressure on tube sheets is not the primary purpose of venting water boxes, C) providing a minimum condensate level in the hot well is not directly related to venting the water boxes, and D) preventing vapor binding of the circulating pump is not the main reason for venting the water boxes.