The correct answer is: C. In the heel slab of a retaining wall, reinforcement is provided at the top of the slab.
The stem of a retaining wall is the vertical portion that supports the earth. The toe slab is the horizontal portion at the bottom of the wall that distributes the load from the stem to the foundation. The heel slab is the horizontal portion at the top of the wall that transfers the load from the stem to the foundation and also helps to resist overturning.
Reinforcement is provided in the stem of a retaining wall near the earth side to resist the tensile stresses caused by the earth pressure. Reinforcement is also provided in the toe slab at the bottom of the slab to resist the shear stresses caused by the earth pressure. However, reinforcement is not provided in the heel slab at the top of the slab because there are no tensile stresses in this region.
Here is a diagram of a typical retaining wall:
The stem is the vertical portion, the toe slab is the horizontal portion at the bottom, and the heel slab is the horizontal portion at the top.
I hope this explanation is helpful!