Over the years, workplace discrimination has become
a. more subtle
b. less subtle
c. more common
d. virtually extinct

The correct answer is A. more subtle.

Over the years, work place discrimination has become more subtle. Subtle means something so precise as to be difficult to analyse or describe.

So basically the work place discrimination has become really unseen or difficult to analyze.

Either there is unnoticeable discrimination in the place or it is so subtle that no one is able to notice it or even if some one does he is not able to understand it.