`S_N1` reaction occurs through the intermediate formation of

Options

A. carbocation
B. carbanion
C. free radical
D. transition

Explanation

To answer the question regarding the intermediate formation in an \( S_N1 \) reaction, we can break down the process step by step.

### Step-by-Step Solution:

1. **Understanding \( S_N1 \) Reaction**:
- The \( S_N1 \) reaction stands for Substitution Nucleophilic Unimolecular. It involves two main steps: the formation of a carbocation intermediate and the nucleophilic attack.

2. **Formation of the Carbocation**:
- In the first step of the \( S_N1 \) mechanism, the leaving group (in this case, a halogen such as chlorine) departs from the substrate (e.g., \( CH_3CH_2Cl \)). This results in the formation of a carbocation.
- For example, when \( CH_3CH_2Cl \) loses the chlorine atom, it forms \( CH_3CH_2^+ \) (a primary carbocation).

3. **Stabilization of the Carbocation**:
- The carbocation formed is often unstable and can rearrange or be stabilized by nearby atoms or groups. However, in the context of \( S_N1 \), we focus on the fact that it is a key intermediate in the reaction.

4. **Nucleophilic Attack**:
- In the second step, a nucleophile (such as bromide ion \( Br^- \)) attacks the carbocation. This step completes the substitution reaction, resulting in the formation of the final product (e.g., \( CH_3CH_2Br \)).

5. **Conclusion**:
- The intermediate formed during the \( S_N1 \) reaction is the **carbocation**. This is the crucial species that allows the nucleophile to attack and complete the substitution process.

### Final Answer:
The intermediate formation in an \( S_N1 \) reaction is a **carbocation**.