Section 8(6)(a) in The Public Liability Insurance Rules, 1991
(a)where the person to be served is a company, if the document is addressed in name of the company, at its registered office or at its principal office or place of business, and is either,-(i)sent by registered post; or(ii)delivered or affixed at some conspicuous part of the premises at its registered office or at the principal office or place of business;