Which of the following is an example of `S_N2` reaction?

Options

A. `CH_3Br+OH^(-)rarrCH_3OH+Br^(-)`
B. `CH_3-underset(Br)underset(|)CH-CH_3OH^(-)rarrCH_3-underset(CH_3)-underset(OH)underset(|)CH-CH_3`
C. `CH_3CH_2OHoverset(-H_2O)rarrCH_2=CH_2`
D. `(CH_3)_3C-Br+OH^(-)rarr(CH_3)_3COH+Br^(-)`

Explanation

To determine which of the following is an example of an `S_N2` reaction, we need to understand the characteristics of `S_N2` reactions.

### Step-by-Step Solution:

1. **Understanding `S_N2` Reactions**:
- `S_N2` stands for bimolecular nucleophilic substitution. This means that the rate of the reaction depends on the concentration of both the substrate (alkyl halide) and the nucleophile.
- It is a second-order reaction.

2. **Identifying Alkyl Halides**:
- `S_N2` reactions are most favorable with primary (1°) alkyl halides. This is because they allow for a backside attack by the nucleophile, which is essential for the mechanism of `S_N2`.

3. **Evaluating the Options**:
- If given multiple options, we need to identify which ones are primary alkyl halides.
- A primary alkyl halide is one where the carbon attached to the halogen is bonded to only one other carbon atom (or is a methyl group).

4. **Analyzing Each Option**:
- For each option provided, determine if it is a primary, secondary (2°), or tertiary (3°) alkyl halide.
- Recall that secondary and tertiary alkyl halides do not favor `S_N2` reactions due to steric hindrance.

5. **Selecting the Correct Answer**:
- After evaluating all options, select the one that is a primary alkyl halide, as this will be the example of an `S_N2` reaction.

### Conclusion:
- The correct answer will be the option that represents a primary alkyl halide.