4. A question or, as the case may be, a reply may be duly served—
(a)where the person to be served is the respondent, by delivering the question to him, or by sending it by post to him at his usual or last known residence or place of business; or
(b)where the person to be served is the person aggrieved, by delivering the reply to him, or sending it by post to him at his address for reply as stated by him in the document containing the questions or, if no address is so stated, at his usual or last known residence; 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 Industrial Relations (N.I.) Order 1992(1), 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.