For a reaction $\text{A}\to \text{B}$, enthalpy of reaction is $-4.2{\text{kJ mol}}^{-1}$ and enthalpy of activation is $9.6{\text{kJ mol}}^{-1}$. The correct potential energy profile for the reaction is shown in option:

Enthalpy of reaction ($\Delta H$) is negative ($-4.2\text{kJ/mol}$), meaning the reaction is exothermic. In such reactions, the potential energy of the products is lower than that of the reactants.Enthalpy of activation ($E_a=9.6\text{kJ/mol}$) is the energy barrier from reactant to transition state.Graph D represents an exothermic profile where the final energy level is lower than the starting level.