A buyer is usually not willing to spend a lot of time and energy researching the market when:

c. savings to be made are small or
d. prices vary but quality is still the same

I read the text but I couldn't really find this exactly, both answers seem like they could be right but obviously one is better than the other. I think its C because if the savings is not substantial people arent going to go to all the trouble, but D Im not sure because if the quality is just as good as the next they might not research but I think people would still try to find the lower price?

Your first thought is correct. If prices vary for the same quality, then people will take the time to search out the lowest price.