Which is not considered a sex-linked trait?
a. purple flower color in pea plants
b. eye color in fruit flies
c. red-green color blindness in humans
d. hemophilia in humans

D. Hemophilia.
The reason it is not a,b or c is because all of them have something to do with something you are born with. Which is genetically transferred.
Example: If your mother has cancer when you are born, you will not be born with cancer. However, if your mother had blue eyes, you could possibly inherit the trait from your mother.

D, Hemophilia
What controls traits and inheritance?
Math. If you look at punnett squares for example, you notice it is simply statistics. However, what literally controls it is what genes are on your chromosomes, and how likely the alleles are to express themselves physically. The controller of traits and inheritance is simply how your alleles chose to align, creating your chromosomes.
Chromosomes

The answer would be d because x,y = male and x,x = female so x,y is male so xxy is male with a female trait!