During an isothermal reversible expansion of an ideal gas against a constant external pressure, which of the following is correct?

For an isothermal process of an ideal gas, internal energy depends only on temperature, so $\Delta U = 0$. From the first law ($\Delta U = q + w$), $q = -w$. Note: Reversible expansion implies $P_{\text{ext}} = P_{\text{int}}$, but the question states 'isothermal reversible', for which $\Delta U = 0$ holds universally.