Answer:
(C) The manufacturer's products, unlike most equally good competing products sold by veterinarians, are also available in pet stores and in supermarkets.
Step-by-step explanation:
Only option (C) explains why very few veterinarians accepted the offer. If the manufacturer offers free promotional materials to all veterinarians clients and all this material is available at other supermarkets and malls their will be good probability that clients will go directly to supermarkets and malls reducing income of veterinarians.