Explanation:

- Citric acid is a natural compound commonly found in citrus fruits.

- Option 1: Onions

- Onions do not contain significant amounts of citric acid.

- Option 2: Apples

- Apples contain malic acid as the main acid, not citric acid.

- Option 3: Oranges

- Oranges are citrus fruits, rich in citric acid.

- Option 4: Bananas

- Bananas contain very little citric acid, primarily having malic acid.