The correct answer is: B. To withstand pressure that increases with depth.
The pressure of water increases with depth. This is because the weight of the water above exerts a force on the water below. The deeper the water, the more water there is above, and the greater the force.
The wall of a dam is broader at the base to withstand this pressure. The wider base distributes the force of the water more evenly, preventing the dam from collapsing.
Option A is incorrect because streamlining is not required for a dam. A dam is not a moving object, so there is no need to reduce drag.
Option C is incorrect because pressure does not increase in a horizontal plane. The pressure of water increases with depth, but it remains constant in a horizontal plane.
Option D is incorrect because pressure is not increased with atmospheric pressure. Atmospheric pressure is the pressure of the air around us. It is a constant force that acts on all objects, regardless of their depth.