In English grammar, a question tag is a short interrogative phrase that is added to the end of a statement. The purpose of a question tag is to seek confirmation, agreement, or to prompt a response from the listener. In the given statement "We need not take the matter so seriously," the main clause is a negative statement indicating that there is no necessity to take the matter seriously. When forming a question tag, you generally take the auxiliary verb from the main clause and reverse its polarity. The auxiliary verb in the statement is "need not." When reversed, it becomes "need." Then, to form the question tag, you add the pronoun "we." So, "need we?" is the appropriate question tag for the given statement. The complete sentence with the question tag is: "We need not take the matter so seriously, need we?"