At a constant temperature in a saturated aqueous salt solution in contact with undissolved salt...

a) addition of a common ion enables more salt to dissolve
b) the rate of dissolving exceeds that of crystallization
c) the rate of dissolving equals the rate of crystallization of the salt
d) the mass of salt in solution equals the mass of undissolved salt
e) all the salt is found in undissolved salt at the bottom of the container

If the solution is saturated, and the temperature (and therefore solubility) is not changing, then the solution is at what is called a "dynamic equilibrium". This means that, although individual particles of the salt are crystallizing and dissolving all the time, the total amount of salt in solution, as well as the total salt that remains undissolved, is constant.

In short, the correct answer is C.