It is true that :

Options

A. A second order reaction is always a multistep reaction
B. A first order reaction is always a single step reaction
C. A zero order reaction is a multistep reaction
D. A zero order reaction is a single step reaction

Explanation

To solve the question regarding the nature of different orders of reactions, we need to analyze each statement provided in the options.

### Step-by-Step Solution:

1. **Understanding Reaction Orders**:
- **Zero Order Reaction**: The rate of reaction is independent of the concentration of the reactants. The rate law can be expressed as \( R = k \), where \( k \) is the rate constant. Increasing the concentration of reactants does not affect the rate of reaction.
- **First Order Reaction**: The rate of reaction is directly proportional to the concentration of one reactant. The rate law can be expressed as \( R = k[A]^1 \). First-order reactions can proceed through multiple steps, especially in complex mechanisms.
- **Second Order Reaction**: The rate of reaction is dependent on the concentrations of two reactants or the square of the concentration of one reactant. The rate law can be expressed as \( R = k[A]^1[B]^1 \) or \( R = k[A]^2 \). Second-order reactions can also be single-step or multi-step depending on the reaction mechanism.

2. **Analyzing Each Statement**:
- **Option 1**: "A second-order reaction is always a multi-step reaction."
- This statement is incorrect. A second-order reaction can be a single-step reaction if it involves two reactants with first-order kinetics or one reactant with second-order kinetics.

- **Option 2**: "A first-order reaction is always a single-step reaction."
- This statement is also incorrect. Many first-order reactions can occur through multiple steps, especially in complex mechanisms like nucleophilic substitutions.

- **Option 3**: "A zero-order reaction is always a multi-step reaction."
- This statement is correct. Zero-order reactions typically cannot proceed in a single step due to their independence from reactant concentration, often requiring multiple steps to proceed.

- **Option 4**: "A zero-order reaction is a single-step reaction."
- This statement is incorrect. As mentioned, zero-order reactions cannot occur in a single step.

3. **Conclusion**:
- The only correct statement is **Option 3**: "A zero-order reaction is always a multi-step reaction."

### Final Answer:
The correct option is **Option 3**: "A zero-order reaction is always a multi-step reaction."