Explanation:

- Spinach contains oxalic acid. It can bind with calcium and other minerals, affecting their absorption in the body.

- Formic acid is found in ant venom and some plants, but not commonly in spinach.

- Lactic acid is produced during fermentation or by muscles under anaerobic conditions, not found in significant amounts in spinach.

- Citric acid is abundant in citrus fruits like lemons and limes, not spinach.

Your answer is option:4- Oxalic acid.

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