The answer is C. heterozygous.
Let's take that A is a dominant allele of the gene, and that a is a recessive allele of the gene.
So, a mother is homozygous dominant (she has both alleles dominant). Let's express it as AA.
A father is homozygous recessive (he has both alleles recessive). Let's express it as aa.
The mother can give only dominant allele A, and the father can give only recessive allele a. So, their offspring can be only Aa (one allele dominant and one allele recessive):
Parents AA x aa
Offspring Aa Aa Aa Aa
