The correct answer is: B. Increased gradually towards the edge.
The thickness of a foundation is increased gradually towards the edge to ensure uniform pressure distribution. This is because the pressure at the edge of the foundation is greater than the pressure at the center. By increasing the thickness of the foundation towards the edge, the pressure is distributed more evenly, which reduces the risk of the foundation sinking.
Option A is incorrect because keeping the thickness of the foundation uniform throughout will result in uneven pressure distribution. This can cause the foundation to sink at the edges.
Option C is incorrect because decreasing the thickness of the foundation towards the edge will also result in uneven pressure distribution. This can cause the foundation to sink at the center.
Option D is incorrect because keeping the thickness of the foundation zero at the edge will not provide any support for the foundation. This can cause the foundation to sink.