They went to the beach _________ a picnic.

Explanation

Correct answer : (a) for

The correct sentence is: “They went to the beach for a picnic.” “For a picnic” is used when we mention the purpose of going somewhere. The sentence structure is: go to + place + for + purpose. “On a picnic” is correct only when we say: They went on a picnic. Since the place (the beach) is already mentioned, for is grammatically correct here.