**Answer: Enthalpy remains constant**

During isothermal expansion of an ideal gas, $\Delta E=0, \Delta T=0$

From the definition of enthalpy,

$H=E+PV$

or $\Delta H = \Delta E + \Delta (PV)$

or $\Delta H = \Delta E + \Delta (nRT)$ {Since, PV=nRT for an ideal gas}

or $\Delta H = \Delta E + nR \Delta T$

or $\Delta H = 0$