Can pressure remain constant in isothermal expansion? An ideal gas undergoes isothermal expansion at constant pressure. During the process:
  
  
  enthalpy increases but entropy decreases.
  enthalpy remains constant but entropy increases.
  enthalpy decreases but entropy increases.
  Both enthalpy and entropy remain constant.
  


I applied this formula.
$\Delta H = \Delta U + p\Delta V$ (pressure is constant)
$\Delta U = 0$ (since, process is isothermal)
$\Delta V &gt;0$ (gas is expanding)
So $\Delta H &gt; 0$
So enthalpy must be increasing $\left(\Delta S =\frac{\Delta Q_{\text{rev}}}{T}\right)$

Since heat is supplied to expand gas $\Delta S$ must be positive. And entropy must increase. But none of the option is matching with my answer.

Also I think question is wrong since $PV= nRT$ and, at constant temperature (isothermal) and constant pressure, volume can not change.
$$\Delta H = \Delta U + \Delta PV$$
Here you were wrong you wrote that 
$$\Delta H = \Delta U + P\Delta V,$$ thus the product of 
 $PV$  remains constant and the $\Delta H$ is zero, so that the enthalpy is constant. And you are wrote that entropy will increase, hence the answer is B.