Concept: The question tests on the area of Probability
We are not selecting a married couple. We are selecting a married employee from B  and thus the probability of such an event is  50/500 =1/10
There will be only 1 partner of the married person in A and the probability of that event is 1/400
This is an AND event(1 married person from A and the partner in B)
=> 50/500* 1/400 =1/4000  (option c)