Really, for X-linked traits, all we can say is that a male with the gene has the trait and a male without the gene doesn't. The situation for the females could be one of a number of things. It could be a matter of strict dominance or strict recessiveness, as Zojojo explained. It could be a partial dominance situation, where a female with one copy of the gene is not as "powerful" magically as a female with two copies of the gene, or it could be a codominance situation where a female with two different kinds of magic genes gets two different kinds of magic. Or, because of the fact that this is carried on the X chromosome, it could do something like calico cats, where (depending on how exactly the magic is generated) heterozygous women could have unpredictable amounts of magic.
Working with codominance, conclusions A and C work perfectly, and B depends on the exact mechanism of magic.
Working with codominance, conclusions A and C work perfectly, and B depends on the exact mechanism of magic.