In which mixture can hand picking be useful?

Correct answer: Gram and small stones

Hand picking is a physical separation method used when an unwanted component is present in a relatively small amount and is large enough, visibly different, and easy to remove by hand. In a mixture of gram and small stones, the stones can be seen because they differ in colour, size, and appearance from the gram; they can therefore be picked out individually. Thus option A is correct. Salt water and sugar water contain dissolved particles that cannot be removed by hand, so evaporation or crystallisation is more suitable. Air is a uniform mixture of gases and its components are invisible, so hand picking cannot separate it. The method depends on visible physical differences.