Answer:
c. All of these are correct
Explanation:
Marketing research refers to assembling, analyzing and processing information, which is used to determine and create the marketing strategies a firm shall follow.
Marketing research refers to analyzing such data which helps a business to create define applicable marketing strategies in a given scenario.
A firm may carry out it's own marketing research or outsource the same function to marketing research companies or advertising agencies.
Hence all three, Marketing research companies, Advertising agencies and companies that produce or sell goods may conduct marketing research.