An Ideal Gas, Consisting of N Moles, Undergoes a Reversible

An ideal gas, consisting of n moles, undergoes a reversible isothermal process during which the volume changes from V_i to V_f. The change in entropy of the thermal reservoir in contact with the gas is given by:

A) nR(V_f - V_i)
B) nR ln(V_f - V_i)
C) nR ln(V_i/V_f)
D) nR ln(V_f/V_i)
E) none of the above (entropy can't be calculated for an irreversible process)