The student has a friend that knows the student is taking biology, and she is confused about her blood type. her blood type is o, but her dad is a and her mother is b. she asks the student if it is possible for her parents to have a child that is o. explain the answer to her.

Blood type is transmitted from parents to children by a Mendelian transmission. Each person has two genotypes of blood type, one from the mother and one from the father,
If the person's blood type is A it means its genotype is AO or AA
If the person's blood type is B it means its genotype is BO or BB
If the person's blood type is O it means its genotype is certainly O

If the parents are AO and BO (it means they have A and B phenotype), it can both transmit their O genotype to their children and he will have OO genotype (so O phenotype). And that was the case for the student.