Brian wants to buy a mare which he can breed to help develop a horse-breeding operation. Larson offers to sell Brian a horse which he believes is healthy, and should make a good broodmare when a little older. After two years, the horse has not yet foaled. The veterinarian tells Brian that the horse is incapable of breeding. If Brian sues to rescind the contract with Larson, it is:

Brian was not aware of that fact that the horse is incapable of breeding at the point he buys it, but Larson assures Brian the horse is healthy. In this light, if Brian sues to cancel the contract with Larson, the court will allow it based on a mistake of fact. This way the court will reduce any civil liability or criminal culpability because Larson might not know that the horse cannot breed, although he is certain that the horse is healthy
Hence, the right answer is "Likely that a court will allow the rescission based on a mistake of fact"