Silk worms feed on mulberry leaves. Mulberry leaves are the sole source of nutrition for silk worms. They are specifically adapted to consume the leaves of the mulberry tree, which provides them with the necessary nutrients to produce silk. Silk worms have a unique digestive system that can break down the tough cellulose in mulberry leaves, allowing them to extract the nutrients and convert them into silk protein. Therefore, option 4, mulberry leaves, is the correct answer. The other options, basil leaves, curry leaves, and rose leaves, are not the preferred food sources for silk worms and would not provide them with the necessary nutrition to produce silk.