Knoevenagel reaction.

Explanation

### Step-by-Step Solution for the Knoevenagel Reaction

**Step 1: Identify the Reactants**
- The Knoevenagel reaction involves an aldehyde and a compound containing an active methylene group, typically a diester. In this case, we use benzaldehyde (C₆H₅CHO) and a diester, such as diethyl malonate (CH₂(COOC₂H₅)₂).

**Hint:** Look for an aldehyde and a compound with an active methylene group (like a diester) as your starting materials.

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