Why might you expect a favorable mutation which is caused by a dominant allele to spread more?

Well this is a poorly worded question. You should probably write to the side of this question "Dear Professor, You are awful at writing tests. Go back to graduate school". That aside, I would go with B in that there is no phenotypic expression of a the recessive allele in a heterozygous individual. C is technically also sort-of true too except it does not answer the question which alludes that there IS natural selection taking place because the mutation in favorable. I can tell you for sure that it is definitely NOT A, D, or E.