a
\(\Delta \mathrm{Q}=\Delta \mathrm{U}+\mathrm{P} \Delta \mathrm{V} \quad[\mathrm{PV}=\mathrm{nRT}\)

\(\Delta \mathrm{Q}=\Delta \mathrm{U}+\mathrm{nR} \Delta \mathrm{T} \quad \mathrm{P} \Delta \mathrm{V}=\mathrm{nR} \Delta \mathrm{T}\)

For isothermal change \(\Delta \mathrm{T}=0\)

\(\therefore \Delta Q=\Delta U\)

In other words whole of heat supplied is converted into internal energy.