In order to know whether "screen" or "screens" should be use here, we need to know what the "that" represents here.
Before I hit the point, what I should mention is "that is" here is the same as "in other words". Then let me try to put the part which has beem eliminated from the sentence back.
Not all employment selection mechanisms that have a "disparate effect", that is (in other words), not all employment selection mechanisms that screen out a percentage of minorities or women disproportionated to their presence in the relevant market, are unlawful.
Now we can see clearly, "that" here represents "employment selection mechanisms". I guess you have known why A is not right wihout any more explainations. | 
