Concept:A trade association is an organisation formed by traders and manufacturers in the same line of business to protect and promote their common interests.
Explanation:Traders and manufacturers who deal in the same product or industry come together voluntarily to form this group.
Their main purpose is to cater for the interests and welfare of all members.
A trade union is for employees or workers, not traders and manufacturers.
A co-operative society is formed by people with common needs but not mainly traders and manufacturers.
A chamber of commerce represents all business persons in a town or city, not just traders and manufacturers.
Therefore, the organisation formed mainly by traders and manufacturers to cater for members' interests is a trade association.
Answer:C. trade association