The correct answer is B) avoid a buildup of pressure within the gear case. Reduction gear casings are vented in order to prevent a dangerous buildup of pressure inside the gear case. This is important for the safe operation of the gears, as excessive pressure could lead to gear damage, oil leaks, or even casing failure. Venting the casing allows any pressure that builds up within the case to be safely released, maintaining the proper operating conditions for the gears. The other answer choices are incorrect because they do not directly address the primary purpose of venting the reduction gear casing, which is to prevent pressure buildup. Windage cooling, lube oil foaming, and axial gear clearance are secondary considerations, but do not constitute the main reason for the venting.