What risk may occur when curd is stored in a copper vessel?

Correct answer: Acids may react with copper and form harmful salts

The relevant concept is the reaction of food acids with reactive metals. Curd contains lactic acid and is therefore acidic. During prolonged contact, acids can react with copper, allowing copper compounds or salts to form and possibly contaminate the food. Excess copper intake can be harmful, so acidic foods are not safely stored in copper or brass vessels unless properly lined. The claims that curd becomes sweet, becomes water, or cannot react are false; hence A is correct.