Which of the following statements is NOT CORRECT?
In an isothermal process,
a. there is perfect thermal equilibrium with environment.
b. there is no change in internal energy during the process.
c. the work done is equal to zero.
d. the time taken is very large.

In an isothermal process, the temperature of the system remains constant, which means $\Delta T = 0$.

For an ideal gas, the internal energy depends only on temperature, so the change in internal energy $\Delta U = 0$.

To maintain a constant temperature, the process must be carried out very slowly so that the system remains in perfect thermal equilibrium with its surroundings. Thus, the time taken for the process is very large.

According to the first law of thermodynamics, $\Delta Q = \Delta U + \Delta W$. Since $\Delta U = 0$, we have $\Delta Q = \Delta W$. The work done in an isothermal process is given by $W = nRT \ln\left(\dfrac{V_2}{V_1}\right)$, which is not zero unless the volume is also constant.

Therefore, the statement that the work done is equal to zero is incorrect.

Answer: c