Never/don't look a gift horse in the mouth. It means to be grateful for a gift instead of asking questions about it or trying to find sth wrong about it and critizeing the gift.

"Look." It comes from the days when ordinary people bought and sold horses. Just as you might check a used car for rust spots or loose ball joints, people knew how to judge the quality of a horse. According to an agricultural college, "The art of determining the age of horses by inspection of the teeth is an old one. It can be developed to a considerable degree of accuracy in determining the age of young horses. The probability of error increases as age advances..." If you were buying a horse, you would inspect the teeth carefully to determine how old the horse was. You would "look the horse in the mouth." In the saying, "a gift horse" means a horse that someone is giving you as a gift, not a horse that you are buying. The saying means that it is rude and improper to judge the quality of a gift. For example, suppose someone gave you an iPhone. It would be polite to say "Oh, how wonderful! Just what I wanted!" It would be impolite to say "Hmmm... I want at least 128GB of storage, let me turn it on so that I can check to see how much you gave me." That would be "looking a gift horse in the mouth."

B is the correct answer, m8. Btw, the whole sentence means to advise someone not to refuse something good that is being offered.

Thanks guys!!!