Let Q and W denote the amount of heat given to an ideal gas and the work done by it in an isothermal process.
In an isothermal process, the temperature remains constant. The internal energy of an ideal gas is a state function that depends on temperature. Hence, change in internal energy is zero and from the first law of thermodynamics: ΔU = Q - W, where ΔU = change in internal energy, Q = amount of heat given and W = work done by it.
Since ΔU = 0 in this case, we get Q = W.
Options (a) and (b) are incorrect because we actually provide a finite amount of heat to the system, and hence work is also not zero.
Option (c) is incorrect because we just showed that Q = W.
Hence, the correct option is option (d).