For a reaction A→B, enthalpy of reaction is −4.2 kJ mol⁻¹ and enthalpy of activation is 9.6 kJ mol⁻¹. The correct potential energy profile for the reaction is shown in option.

The correct potential energy profile for the reaction is shown in Option 2 (represented by the second image, where the potential energy of the product B is lower than that of the reactant A).

To determine the correct potential energy profile, we analyze the given thermodynamic parameters:

1. Enthalpy of Reaction ($\mathrm{\Delta H}$):
The enthalpy of reaction is given as:
$\mathrm{\Delta H}=-4.2\text{kJ}{\text{mol}}^{-1}$
Since the value of $\mathrm{\Delta H}$ is negative ($\mathrm{\Delta H}<0$), the reaction is exothermic. For an exothermic reaction, heat is released, meaning the potential energy of the products ($H_{\text{B}}$) is lower than the potential energy of the reactants ($H_{\text{A}}$):
$\mathrm{\Delta H}=H_{\text{products (B)}}-H_{\text{reactants (A)}}<0$
$\Rightarrow H_{\text{B}}<H_{\text{A}}$
Thus, on the potential energy diagram, the energy level of product B must lie below the energy level of reactant A.

2. Enthalpy of Activation ($E_a$):
The enthalpy of activation (activation energy) is given as:
$E_a=9.6\text{kJ}{\text{mol}}^{-1}$
The activation energy represents the minimum energy barrier that the reactants must overcome to reach the transition state at the peak of the curve. This is a positive value, indicating that the potential energy increases from A to the peak, and then decreases as it forms B.

Comparing the Options:
- Option 1 shows reactant A at a lower potential energy level than product B ($H_{\text{B}}>H_{\text{A}}$), which represents an endothermic reaction.
- Option 2 shows reactant A at a higher potential energy level than product B ($H_{\text{B}}<H_{\text{A}}$), which correctly represents an exothermic reaction.
- Option 3 shows A and B at approximately the same energy level.
- Option 4 shows reactant A at a much lower potential energy level than product B.

Therefore, the profile corresponding to Option 2 is correct.