in response to 'a': if one were to stop the muscle building exercise, it is unlikely that one would remain the same, though it is possible.

in response to 'b': if one were to stop exercising and reduce calorie intake to amount it was before exercising, this is very likely (actually, for a while, the person could take in more calories than before while still having more muscle mass, at least. there's probably an additional benefit from exercise that makes insulin absorbtion more efficient after a period of exercise). if one stopped exercising, yet maintained the same amount of calories, that person would get fat. maybe this is what you're thinking of. if so, that's quite different than saying muscle will turn to fat. calories can contribute to the building of muscle, or the storage of fat. in order for the building of muscle to occur, that muscle has to be stimulated to grow (fat does not need to be so stimulated). muscle and fat are different.

in response to 'c': muscle does not turn into fat.