A person with type A blood (unknown genotype) marries a person with type O blood. What blood types are possible among their children. (Show 2 crosses)

If a person with blood type A, of unknown genotype, and a person with blood type O have offspring, the only possible blood types in children are A and O.

Explanation:

It should be noted that in the case of blood type A, the possible genotypes are A|A or A|O, while blood type O (recessive) will always have genotype O|O.  Assuming that it is the father who has the blood group A:

♂ A|A or A|O

♀ O|O

- Cross 1: If the father possesses the dominant A characteristic on both alleles

Alleles     A         A

O           A|O      A|O

O          A|O       A|O

In this case all the offspring will have a blood type A, with a heterozygous genotype.

- Cross 2: If the father has blood type A, but with a heterozygous genotype

Alleles     A         O

O            A|O     O|O

O            A|O     O|O

50% of the descendants will be of blood type A, heterozygous

The other 50% will be blood type O.

Blood type is determined by the presence or absence of surface antigens A and B.

Thus, depending on the genotype of one of the parents A|A or A|O, when crossed with a parent type O O|O, their children may have blood type A or O.

Learn more in:

Inheritance - ABO blood group link

the answer to your question is:

c. complementary base-pairing