In this case you have two different genes happening at once.
The tricky part is the dimpled gene. We don't know if the boy is
homozygous (DD) or heterozygous (Dd). Clearly ONE of his parents has
to be at least heterozygous, but we don't know which one. His mom
could be any of DD, Dd, or dd. Luckily, we can still knock out
answers b and c - since this gene is NOT sex-linked, the answers XD
and Xd are clearly not correct.
The hemophilia IS sex linked, and the boy's mother has to be either
heterozygous or homozygous for the recessive hemophilia version. Sex
linked genes are described as XH or Xh (usually the allele is a
superscript of the X). So, the mother has a genotype of either XH Xh
or Xh Xh.
Because of the way sex-linked genes are noted, the answer has to be a.
If hemophilia were not sex-linked, it would be different.
An example of crosses using sex-linked genes (specifically hemophilia,
lucky you) is at http://www.ksu.edu/biology/pob/genetics/mod5_class3_problems.htm
(most of the way down the page). That link also will be helpful for
your other questions - and the page has multiple links to other
problems to try out and get instant feedback. I found it by doing a
Google search for "genetic cross mendelian sex linked genotype".
I hope this answer helps you understand where you went astray on your
test. Good luck next time, and please let us know if we can help