The correct answer is: A. A key.
A key is a portion of slab that is constructed downwards at the end of the heel slab. It is designed to increase the factor of safety against sliding. A key is typically constructed in concrete and is reinforced with steel bars.
A cut-off wall is a vertical wall that is constructed to prevent the movement of soil. It is typically constructed in concrete or masonry. A rib is a horizontal beam that is constructed to support the weight of the soil. It is typically constructed in concrete or steel.
In case the factor of safety against sliding is less than 1.5, a portion of slab is constructed downwards at the end of the heel slab, which is known as a key. The key is designed to increase the factor of safety against sliding by providing a mechanical interlock between the heel slab and the foundation soil. The key also helps to distribute the load from the heel slab more evenly over the foundation soil.