The process with negative entropy change is:

🧠 Entropy decreases when gas moles decrease or when molecules become more ordered Check each process for the direction of disorder change.

🗺️ Checking each option (a) $\ce{N2(g) + 3H2(g) -> 2NH3(g)}$: 4 mol gas → 2 mol gas, $\Delta n_g = -2$, fewer gas molecules → $\Delta S < 0$ ✓

(b) Dissolution of iodine in water: solid dissolves into solution, more dispersed → $\Delta S > 0$ ✗

(c) $\ce{CaSO4(s) -> CaO(s) + SO3(g)}$: gas is produced → $\Delta S > 0$ ✗

(d) Sublimation of dry ice ($\ce{CO2}$): solid → gas → $\Delta S > 0$ ✗

Only option (a) gives $\Delta S < 0$.

⚠️ Trap: Don't mistake "the reaction requires energy" for "entropy decreases". Ammonia synthesis is exothermic (favours products enthalpy-wise) and also has $\Delta S < 0$ — both factors must be weighed for spontaneity.

$\boxed{\text{Answer: (a)}}$