Reactant A converts to product D through the given mechanism (with the net evolution of heat): A - B slow; H = +ve B -…

Step 1 — Analyse each step on energy diagram

• A → B (slow, $\Delta H = +ve$): endothermic, B higher than A; slow = large activation energy hump
• B → C (fast, $\Delta H = -ve$): exothermic, C lower than B; fast = small hump
• C → D (fast, $\Delta H = -ve$): exothermic, D lower than C; fast = small hump

Step 2 — Net reaction

Net: $\Delta H_{\text{overall}} < 0$ (net evolution of heat) → D is at lower energy than A

Step 3 — Profile shape

A → rises steeply to B (large hump, slow step) → drops to C (small hump) → drops to D (small hump); D below A.

This matches Graph (a) → Option (a)

Key Points to Remember:

• Slow step = large activation energy barrier on energy diagram
• Endothermic step: products higher than reactants
• Net exothermic: final product D lower than starting material A