The correct answer is A. Hard.
Rubble masonry is a type of masonry that uses uncut or roughly shaped stones. The stones are laid in a random pattern and are held together with mortar. The stones used in rubble masonry must be hard enough to withstand the weight of the structure and the elements. They must also be tough enough to resist cracking and breaking.
Option B, tough, is also important for rubble masonry stones. However, hardness is more important than toughness. A stone that is tough but not hard may still break under the weight of the structure.
Option C, heavy, is not as important for rubble masonry stones. The weight of the stones helps to hold the structure together, but the stones do not need to be very heavy.
Option D, light, is not a desirable quality for rubble masonry stones. Light stones are more likely to break under the weight of the structure.
In conclusion, the stone suitable for rubble masonry should be hard.