The correct answer is B) avoid a buildup of pressure within the gear case. Reduction gear casings are vented to prevent a dangerous buildup of pressure inside the gear case. This is necessary because the gears moving at high speeds can generate heat and cause the air inside the case to expand, potentially leading to an increase in internal pressure. Venting the casing allows this excess pressure to be released, preventing damage to the gear case or other components. The other options are incorrect because they do not accurately describe the primary purpose of venting the reduction gear casing. Option A is incorrect as windage is not the main reason for venting. Option C is incorrect as venting is not primarily to minimize lube oil foaming. Option D is incorrect as venting is not to allow for axial clearance between the gears.