Why do bears hibernate during the winter months?

Bears hibernate during the winter months primarily because their food supply becomes scarce or completely disappears. Therefore, the correct answer is:

B. Bears bypass winter when their food supply disappears to save energy.

During hibernation, bears reduce their metabolic rate and body temperature, which allows them to conserve energy and survive for several months without food. By hibernating, bears can avoid the harsh winter conditions when their food sources are scarce, and wait for more favorable conditions to resume their normal activities in the spring.