Final answer:
Based on ABO blood type inheritance, Bob with blood type O cannot be the father of an AB child, making Carl with blood type A the only possible father.
Step-by-step explanation:
The student's question is about determining the paternity of a child based on ABO blood type inheritance. Susan and her daughter both have blood type AB. Given that blood type AB can only be formed by inheriting A allele from one parent and B allele from another, or receiving AB from each parent, and considering that Bob has blood type O (genotype ii), he could not contribute the A or B allele necessary to result in a child with blood type AB. Therefore, Carl with blood type A (possible genotypes AA or AO) must be the father because he can contribute the A allele.
As an example, according to the information referenced, a male with blood type A and a female with blood type B can have children with blood types A, AB, B, and O. Since Susan has blood type AB and Bob has type O (which means his genotype is ii with neither A nor B alleles to contribute), the only possible father based on this scenario is Carl with blood type A.
Furthermore, the biological impossibility of a type O parent contributing to a type AB child solidifies the answer. Bob with type O blood cannot be the father of Susan's AB type daughter, leaving Carl with type A blood as the only possible father.