The correct answer is B) allow turning with a wrench. Heavy-duty screwdrivers with a square shank are designed to be used with a wrench, which can provide more torque and leverage for turning the screwdriver. The square shank helps prevent the screwdriver from slipping or rounding out when significant force is applied through the wrench. This is particularly useful for loosening or tightening large, stubborn screws or bolts that require more torque than can be easily applied by hand. The other answer choices are incorrect because they do not directly explain the purpose of the square shank. Option A is incorrect as the square shank is not primarily to prevent the shank from bending. Option C is incorrect as the square shank is not meant for striking with a hammer. Option D is incorrect as the square shank is not intended to be used as a pry bar.