Answer:
She must be heterozygous for blood type B.
Monohybrid cross
Explanation:
See the two options of punnet squares attached.
The only possible way they could have a child with a type A phenotype is if the mother is heterozygous (BO), where they have a 1:4 chance.
This type of square is called a monohybrid cross, as we are looking at 1 trait
