A woman has blood type a mm. her child has blood type ab mn. the blood types of the potential fathers are given below.

The mother has the alleles A and M, given that her blood type is A MM. Since the child has blood type AB MN, then the child has the alleles A, B, M, N. However, the alleles B and N, cannot have come from his/her mother and must have come from the father. Therefore, we are looking for a father with the alleles B and N. Based on this conclusion, the child's father is either Tom (with blood type AB MN) or Bill (with blood type B MN).