Final answer:
Each pregnancy has an independent 50% chance of resulting in the birth of a boy, regardless of the genders of any previous children, as this probability is solely determined by the father's contribution of either an X or Y chromosome.
Step-by-step explanation:
The question is about determining the probability that the next child in a family of two girls will be a boy. The key piece of information here is that the sex of a baby is determined by the chromosomes it inherits: XX for a girl and XY for a boy. The chromosome from the mother is always X, while the chromosome from the father can be either X or Y. Thus, for every pregnancy, there is a 50% chance of having a boy and a 50% chance of having a girl because the father has a 50% chance of passing on his Y chromosome and a 50% chance of passing on his X chromosome.
This probability stays constant for each pregnancy, regardless of the genders of previous children. Therefore, the answer to the question is B. 50% for the couple's next child to be a boy.