Biology problem help?

When I was teaching this topic, I would ask everyone to look around, and tell me how many have 6 digits on either a hand or foot. Polydactyly (having extra digits) has several different causes, but one of them is the presence of a dominant allele. So, this dominant allele is not more common in the population. 

What ultimately determines whether a dominant phenotype is more common in the population is selection.

No. Consider a dominant gene which is lethal in its homozygous form but selected for in the heterozygous form. It could not possibly be more prevalent.