Incorrect Statement: (c) a zero order reaction must be a complex reaction

Explanation:
1. Second Order Reaction:
- A second order reaction refers to a reaction whose rate is determined by the concentration of two reactant molecules.
- It can be a bimolecular elementary reaction or a complex reaction involving multiple steps.
- In a bimolecular elementary reaction, two reactant molecules directly collide and react to form the product.
- Hence, statement (a) is correct as a second order reaction can be a bimolecular elementary reaction.

2. Bimolecular Elementary Reaction:
- A bimolecular elementary reaction is a reaction that occurs in a single step through the collision of two reactant molecules to form the product.
- It can be a second order reaction or a higher order reaction.
- Hence, statement (b) is correct as a bimolecular elementary reaction can be a second order reaction.

3. Zero Order Reaction:
- A zero order reaction refers to a reaction in which the rate of the reaction is independent of the concentration of the reactant(s).
- It can be a simple reaction or a complex reaction involving multiple steps.
- In a simple zero order reaction, the rate is independent of the concentration of the reactant(s) as the reaction proceeds at a constant rate.
- Hence, statement (c) is incorrect as a zero order reaction can be a simple reaction, not necessarily a complex reaction.

4. First Order Reaction:
- A first order reaction refers to a reaction whose rate is determined by the concentration of a single reactant molecule.
- It can be a simple reaction or a complex reaction involving multiple steps.
- In a simple first order reaction, the rate is directly proportional to the concentration of the reactant.
- In a complex first order reaction, the overall reaction may involve multiple steps, but the rate-determining step is a first order reaction.
- Hence, statement (d) is correct as a first order reaction can be either a simple or a complex reaction.

Summary:
- The incorrect statement among the given options is (c) a zero order reaction must be a complex reaction.
- Zero order reactions can be simple reactions in which the rate is independent of the concentration of the reactant(s).
- First order reactions can be either simple or complex reactions depending on the nature of the reaction and the rate-determining step.