Correct Option: A

Everyone has two copies of the gene for blood type (and for most other genes as well). Since O is a recessive trait that means that someone with type O blood must have two copies of the O gene. Types A and B are dominant, and since they’re co-dominant, someone with type AB blood must have the A gene and the B gene. In reproduction, the offspring get one copy of each gene from each parent. Since the type O parent has two copies of the O gene, it’s assured that the offspring will get one of them. As for the type AB parent, the offspring has a 50/50 chance of getting the A gene or the B gene. The two possible outcomes are AO and BO. Since O is recessive and A and B are dominant, that means the possible blood types are A and B.