Heat is given to an ideal gas in an isothermal process. A. Internal...

From the first law of thermodynamics, heat supplied to the system $Q$, change in internal energy of the system $∆ U$ and work done by the system $W$ are related as:

$Q = ∆ U + W$

Since the process is isothermal, the temperature of the system is constant and therefore $∆ U = 0$ (i.e., internal energy is constant). 

$⇒ Q = W$

As heat is supplied to the system, $Q$ is positive and therefore $W$ is positive i.e., gas will do positive work.

Hence, correct options are C and D.