Concept:A trade association is an organisation formed by businesses in the same trade or industry to protect and promote their common interests.Explanation:Trade associations do not carry out government functions such as providing infrastructure for members.They also do not provide employment or pay salaries to members.Members join voluntarily to work together on industry problems and to safeguard their shared concerns.Therefore, the main function of a trade association is to promote the welfare of its members.This matches the correct option given in the question.Answer:C. promote members' welfare.