It is true that :

Solution

Chemistry ma’am · 3 steps

Ask whether a single step could ever give that order: for zero order, it never can.

1. Elementary step rule. In a single-step (elementary) reaction, the order equals the molecularity, the number of particles that collide in that step. Molecularity is always 1, 2 or 3. It can never be zero.
2. Zero order. So a zero order reaction cannot be one elementary step. Its rate is set by something other than the reactant concentration, such as a saturated catalyst surface, as in 2NH₃ → N₂ + 3H₂ on hot platinum. That makes it a complex, multistep reaction.

   Option	Claim	Verdict
   (a)	First order is always single step	False: 2N₂O₅ → 4NO₂ + O₂ is first order but goes through several steps
   (b)	Zero order is multistep	True: molecularity cannot be zero, so no single step gives zero order
   (c)	Zero order is single step	False: this would need a molecularity of zero
   (d)	Second order is always multistep	False: a bimolecular step such as NO + O₃ → NO₂ + O₂ is second order in one step

3. So. The only true statement is (b): a zero order reaction is a multistep reaction.

The idea to keep. Order is found by experiment; molecularity belongs to one step and is 1, 2 or 3. If the order is zero or a fraction, the reaction cannot be elementary.

Answer from the NTA final answer key. Solution by PrepWiser.