6. For a reversible process, necessary condition is

(a)  In the whole cycle of the system, the loss of any type of heat energy should be zero

(b)  That the process should be too fast

(c)   That the process should be slow so that the working substance should remain in thermal and mechanical equilibrium with the surroundings

(d)  The loss of energy should be zero and it should be quasistatic

Answer-d