Section 8(2)(b) in The Foreign Contribution (Regulation) Rules, 2011
(b)by sending it to him by 'registered post with acknowledgement due' or 'speed post' to the address of his last known place of residence or the place where he carries on, or is known to have last carried on, business or the place where he personally works for gain or is known to have last worked for gain and, in case the person is an organisation or an association, to the last known address of the office of such organisation or association; or