Acetic acid exists as dimer in benzene due to: A. Condensation reaction B. Hydrogen bonding C. Presence of carboxyl group D. Presence of hydrogen atom at α-carbon

To solve the question about why acetic acid exists as a dimer in benzene, let's analyze the given options one by one:

1. **A. Condensation reaction**:
- A condensation reaction typically involves the joining of two molecules with the loss of a small molecule, often water. While acetic acid can undergo condensation reactions, this is not the primary reason it forms dimers in benzene.

2. **B. Hydrogen bonding**:
- Acetic acid (\(CH_3COOH\)) contains a carboxyl group (\(-COOH\)) that can form hydrogen bonds. When two acetic acid molecules come close to each other, the hydrogen atom of one carboxyl group can form a hydrogen bond with the oxygen atom of the other carboxyl group. This interaction leads to the formation of a dimer.

3. **C. Presence of carboxyl group**:
- The carboxyl group is indeed responsible for the ability of acetic acid to dimerize, as it allows for hydrogen bonding. However, this option is somewhat vague compared to the specific mechanism of hydrogen bonding.

4. **D. Presence of hydrogen atom at \(\alpha\)-carbon**:
- The \(\alpha\)-carbon refers to the carbon atom adjacent to the functional group (the carbon in the carboxyl group). While the presence of hydrogen on the \(\alpha\)-carbon can influence certain chemical reactions and stability, it is not directly responsible for the dimerization of acetic acid in benzene.

### Conclusion
The correct answer is **B. Hydrogen bonding**.

**Explanation**: Acetic acid dimerizes in non-polar solvents like benzene due to the hydrogen bonding between the hydroxyl (\(-OH\)) group of one acetic acid molecule and the carbonyl (\(C=O\)) group of another acetic acid molecule, forming a stable dimer. This dimerization occurs because the hydrogen bonds are strong enough to overcome the tendency of the acetic acid molecules to remain as monomers in a non-polar solvent.