The correct answer is D. Retaining walls.
A heavy stone is suitable for retaining walls because it is strong and can withstand the weight of the soil behind it. Retaining walls are used to hold back soil or water, and they need to be strong enough to do this job. A heavy stone is a good choice for this because it is dense and has a high compressive strength.
Arches are also made of stone, but they do not need to be as strong as retaining walls. Arches are designed to support their own weight, and they do not need to withstand the weight of soil or water. Rubble masonry is a type of masonry that uses small, irregular stones. It is not as strong as a retaining wall or an arch, but it is a good choice for walls that do not need to be very strong. Roads are made of a variety of materials, including stone, concrete, and asphalt. Stone is not the most common material used for roads, but it can be used in some cases.
In conclusion, a heavy stone is suitable for retaining walls because it is strong and can withstand the weight of the soil behind it.