The correct answer is C. 2.2.
Specific gravity is the ratio of the density of a substance to the density of a reference substance. The reference substance is usually water, which has a density of 1 gram per cubic centimeter.
A good building stone should be strong and durable. It should also be able to withstand the elements, such as wind, rain, and snow. Stones with a specific gravity of 2.2 or greater are generally considered to be good building stones.
Stones with a specific gravity of less than 2.2 are not as strong or durable as stones with a specific gravity of 2.2 or greater. These stones are more likely to crack or break under stress.
Therefore, the correct answer is C. 2.2.