The other sets are:

(a) cow, milk, butter. It is related as cow gives milk and butter can be obtained from milk.

(b) hen, meat, egg. Both meat and eggs can be obtained from hen.

(c) goat, milk, meat. Both milk and meat can be obtained from goat.

(d) plants, vegetables, buttermilk. This option is incorrect. Buttermilk is a diary product and can be obtained from milk. Vegetables are obtained from plants but not the milk.