Explanation

(d) In an ideal gas, the internal energy depends only upon the temperature of the gas. When an ideal gas undergoes an isothermal change, there is no change in its internal energy $(\mathrm{\Delta}U = 0)$
From first law of thermodynamics
$\mathrm{\Delta}U = Q - W$
For isothermal change $\mathrm{\Delta}U = 0$
$\therefore\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ Q = W$
Hence, in an isothermal process in an ideal gas the heat absorbed by the gas is entirely used in the work done by the gas.