They should have worked hard.

The question asks for the correct question tag for the statement 'They should have worked hard.' A question tag is a short question added to the end of a statement. The general rule dictates that if the statement is positive, the tag is negative, and vice-versa. The auxiliary verb in the main statement determines the auxiliary in the tag. Here, the primary auxiliary verb is 'should'. Since the statement 'They should have worked hard' is positive, the tag must be negative. The subject 'They' is retained in the tag. Therefore, the correct question tag is 'Shouldn't they?'. Option (B) 'Haven't they?' is incorrect because 'have' is part of the perfect infinitive, but 'should' is the modal auxiliary governing the verb phrase. Option (C) 'passed for' is entirely irrelevant to question tags.