If Huntington's disease is due to a dominant trait, shouldn't three-fourths of the population have Huntington's while one-fourth has the normal phenotype?

answer is (A)

This would be probably true if the assumption that all possible genotypic variations would be equally distributed (so we would have 25% HH, 25% hh and 2x 25 Hh). If this distribution would be true and Huntingtons disease really was a single gene dominant trait diesase, then yes, we could expect such a distribution in the population.