The potential energy profile for a hypothetical reaction X arrow Y...

From the given potential energy profile:

Enthalpy of reactant $X$, $H_X = 40\text{ kJ mol}^{-1}$
Enthalpy of product $Y$, $H_Y = 90\text{ kJ mol}^{-1}$

The enthalpy change ($\Delta H$) for the reaction is given by the difference in potential energy between the products and the reactants:

$\Delta H = H_{\text{products}} - H_{\text{reactants}}$
$\Delta H = H_Y - H_X$
$\Delta H = 90 - 40 = +50\text{ kJ mol}^{-1}$

Answer: $+50\text{ kJ mol}^{-1}$