1. Interpret the mechanism

The reaction proceeds as: $A \to B \quad \text{slow}, \quad \Delta H > 0$ $B \to C \quad \text{fast}, \quad \Delta H < 0$ $C \to D \quad \text{fast}, \quad \Delta H < 0$

Also, the overall reaction evolves heat, so the net enthalpy change is negative.

2. Translate this into an energy profile

For a multistep reaction coordinate diagram:

• Each step has a transition state (peak).
• Since there are 3 elementary steps, there must be 3 peaks.
• Since the first step is slow, it has the largest activation energy. So the first peak must be the highest relative to its preceding level.
• Since the first step is endothermic ($\Delta H > 0$), the intermediate $B$ must lie above $A$ in energy.
• Since the second and third steps are exothermic ($\Delta H < 0$), the intermediate $C$ must lie below $B$, and $D$ must lie below $C$.
• Since the overall reaction is exothermic, final product $D$ must lie below the initial reactant $A$.

So the required energy sequence is: $E_B > E_A, \qquad E_C < E_B, \qquad E_D < E_C, \qquad E_D < E_A$ with three maxima and the first barrier largest.

3. What the correct graph must look like

The correct potential energy diagram should therefore show:

1. Start at $A$.
2. Rise to the highest peak (slow first step).
3. Drop to intermediate $B$, which is still above $A$.
4. Rise to a smaller second peak.
5. Drop to intermediate $C$, which is below $B$.
6. Rise to a third smaller peak.
7. Drop to $D$, which is lowest overall and below $A$.

4. Match with options

Among the given choices, the diagram satisfying all these conditions is Option D.

5. Final answer

Therefore, the correct representation is: $\boxed{\text{D}}$

6. Comparison with stored answer

Stored correct answer: D

My derived answer: D

So, they agree.