Allium cepa belongs to family Solanaceae.

Explanation

**Step-by-Step Solution:**

1. **Identify the Plant**: The question states that Allium cepa is being classified under a specific family. Allium cepa is commonly known as onion.

2. **Understand the Family Classification**: The family mentioned in the question is Solanaceae. This family is known for plants like potatoes, tomatoes, and eggplants.

3. **Correct Family Identification**: It is important to verify the correct family for Allium cepa. Upon research, it is found that Allium cepa actually belongs to the family Amarylidaceae, not Solanaceae.

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