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 determine which statement is correct regarding the order of reactions, we need to analyze each of the provided statements about reaction orders step by step.

### Step 1: Analyze the First Statement
**Statement 1:** "The second order reaction is always multi-step."

- **Explanation:** This statement is incorrect. A second-order reaction can occur in a single step or in multiple steps. For example, an SN2 reaction is a second-order reaction that occurs in a single step where the nucleophile attacks the substrate directly.

### Step 2: Analyze the Second Statement
**Statement 2:** "The first order reaction is always in a single step."

- **Explanation:** This statement is also incorrect. A first-order reaction can occur in multiple steps. For instance, in an SN1 reaction, the formation of a carbocation is a separate step, and then the nucleophile attacks in another step. Thus, it is not always a single-step reaction.

### Step 3: Analyze the Third Statement
**Statement 3:** "The SN1 reaction is a zero-order reaction and is always in a single step."

- **Explanation:** This statement is incorrect. The SN1 reaction is a first-order reaction, not a zero-order reaction. It involves multiple steps: the formation of a carbocation followed by nucleophilic attack. Therefore, it cannot be classified as a zero-order reaction.

### Step 4: Analyze the Fourth Statement
**Statement 4:** "The decomposition of HI is a zero-order reaction and is always multi-step."

- **Explanation:** This statement is correct. The decomposition of hydrogen iodide (HI) can be classified as a zero-order reaction, and it typically occurs through multiple steps. In zero-order reactions, the rate does not depend on the concentration of the reactants, and the reaction can proceed through various intermediates.

### Conclusion
Based on the analysis of all four statements, the correct statement is:
**Statement 4:** "The decomposition of HI is a zero-order reaction and is always multi-step."