Concept:A certificate of trading is a licence that allows a company to start business operations after incorporation.Explanation:A certificate of incorporation only confirms that a company has been legally registered.Before a public company can begin trading, it must obtain a certificate of trading from the appropriate authority.This certificate proves that the company has met all legal conditions for commencing business.Private companies, sole proprietorships and co-operative societies do not need a certificate of trading for this purpose.Answer:D. public company