6. A question and any reply thereto may be served on the respondent or, as the case may be, on the person aggrieved—
(a)by delivering it to him; or
(b)by sending it by post to him at his usual or last-known residence or place of business; or
(c)where the person to be served is a body corporate or is a trade union or employers' association within the meaning of the Trade Union and Labour Relations Act 1974, by delivering it to the Secretary or clerk of the body, union or association at its registered or principal office or by sending it by post to the secretary or clerk at that office; or
(d)where the person to be served is acting by a solicitor, by delivering it at, or by sending it by post to, the solicitor's address for service; or
(e)where the person to be served is the person aggrieved, by delivering the reply, or sending it by post, to him at his address for reply as stated by him in the document containing the questions.