Benzaldehyde undergoes aldol condensation in an alkaline medium.

Explanation

**Step-by-Step Solution:**

1. **Understanding Aldol Condensation**: Aldol condensation is a reaction between two aldehydes or ketones that have alpha hydrogen atoms. In this reaction, one molecule acts as a nucleophile and the other as an electrophile, leading to the formation of a β-hydroxy aldehyde or ketone, which can further dehydrate to form an α,β-unsaturated carbonyl compound.

2. **Identifying Benzaldehyde**: The chemical structure of benzaldehyde is represented as C6H5CHO. It consists of a phenyl group (C6H5) attached to a carbonyl group (CHO).

3. **Checking for Alpha Hydrogen**: To undergo aldol condensation, a compound must have at least one alpha hydrogen atom (hydrogens attached to the carbon adjacent to the carbonyl group). In the case of benzaldehyde, the carbon adjacent to the carbonyl (C=O) is part of the phenyl ring and does not have any hydrogen atoms attached to it.

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