Why is it chemically unsafe to store curd or pickle in a copper vessel?

Correct answer: Their acids can form harmful salts with copper

Curd contains lactic acid, and pickles commonly contain acids such as acetic acid or citric acid. The governing principle is that acids can react with reactive metals, including copper, especially during prolonged contact. Such reactions may form soluble copper compounds that contaminate food and can be harmful in excess. Therefore option A is correct; the other options deny acidity or describe impossible transformations.