B. O, A ruled out.
Mother is B blood type.
Thymine is made by a series of steps. The mutants could be blocked at any step in the pathway and get the same phenotype. The two mutations have complemented, so they must affect different polypeptides which are either part of the same enzyme of parts of different enzymes. C. 2 closely linked genes. Since there are no recombinants, there is no need to posit two genes.