`SN` reaction is given by these compounds, which have a nuclophilic group and a good leaving `EWG`. It should be stable after leaving with bonding pair of `overline(e)'s` and it should have high polarisability.
Nucliphilic aliphatic substituion reaction is mainly of two types `SN^(-1)` and `SN^(2). SN^(-1)` mechanism is a two step process. Reaction velocity of `SN^(-1)` depends only on the concentration of the subtrate. It proceeds via the formation of carbocation, optically active substrate gives `(o+)` and `(o-)` froms of the product.
In most of the cases, the product usually consists of `5-20%` inverted and `(95-80%)` racemised species. The more stable is the carbocation, the greater is the propotion of racemission. In solvolysis reaction, the more nucleophillic is the solvent, the greater is the proportion of inversion.
Which of the following gives `SN^(1)` reaction?

Options

A.
B.
C. `MeBr`
D. All

Explanation

To determine which compound gives an SN1 reaction, we need to analyze the stability of the carbocations formed during the reaction. The SN1 mechanism involves the formation of a carbocation intermediate, and the stability of this carbocation is crucial for the reaction to proceed via this pathway.

### Step-by-Step Solution:

1. **Understanding SN1 Mechanism**:
- The SN1 reaction is a two-step process where the first step involves the formation of a carbocation. The stability of this carbocation is key to whether the reaction will proceed via the SN1 mechanism.

2. **Identify the Compounds**:
- We need to analyze the given compounds (let’s denote them as A, B, and C) to see which one can form a stable carbocation.

3. **Analyze Compound A**:
- Compound A forms a carbocation that is resonance stabilized due to the presence of a phenyl ring and two methyl groups. The positive charge on the carbocation can be delocalized through resonance, making it highly stable.
- **Conclusion**: Compound A is likely to undergo an SN1 reaction due to the stability of its carbocation.

4. **Analyze Compound B**:
- Compound B is a primary alkyl halide. When it forms a carbocation, it results in a primary carbocation, which is not stable.
- **Conclusion**: Compound B will not undergo an SN1 reaction; instead, it will favor an SN2 reaction.

5. **Analyze Compound C**:
- Compound C forms a methyl carbocation upon leaving the leaving group. Methyl carbocations are highly unstable.
- **Conclusion**: Compound C will also not undergo an SN1 reaction and will favor an SN2 mechanism.

6. **Final Decision**:
- Based on the analysis, only Compound A forms a stable carbocation and is capable of undergoing an SN1 reaction.

### Answer:
The compound that gives an SN1 reaction is **Compound A**.