A question to the respondent may be served on him—
(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 acting by a solicitor, by delivering it at, or by sending it by post to, the solicitor’s address for service.
(d) 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 (Consolidation) Act 1992 by delivering it to the secretary or clerk of the body, union or association at its registration or principal office or by sending it by post to the secretary or clerk at that office.