Answer: c. The manufacturer's products, unlike most equally good competing products sold by veterinarians, are also available in pet stores and in supermarkets.
Explanation:
It is indeed irregular that the Vets would refuse to sell the product especially since they were offered incentives and it is stated that Veterinarians generally derive some of their income from selling several manufacturers’lines of petcare products. .
Why then would they do this?
Looking at the options, option C would be the best option.
If the product was being sold in supermarkets already, that is competition that Veterinarians will much rather avoid seeing as they have similar high quality products that they don't have to compete with supermarkets to sell.
It would make sense to just keep selling the ones they are selling than to get into undue competition.