A woman has blood Type A, her baby has blood Type AB: Is it possible that a man who has the blood Type A could be the father? Show the Punnett Square and explain.

No, it is not possible

Explanation:

Humans use the ABO blood group system where the A and B alleles are dominant over allele O but codominant with one another.

This system is such that;

Type A can only have genotype: iAiA or iAi

Type B can only have genotype: iBiB or iBi

Type AB can only have genotype: iAiB

Type O can only have genotype: ii

According to this question, a woman has blood Type A (iAiA or iAi) and her baby has blood Type AB (iAiB). This means that the father definitely contributed the allele B in order to have a baby with type AB. It also means that, according to the question, IT IS NOT POSSIBLE FOR A "Type A" father to produce that baby (see the punnet square attached).

Note: The father must contain an allele B in his genotype i.e. the father must be either type B (iBiB or iBi) or type AB (iAiB)