Answer: B 
Country b has a comparative advantage in producing DVDs 
Comparative advantage is advantage enjoyed by that country or a business that is able to produce a good or a service at a lower opportunity cost than the other. 
In the given example, we can see that country A will have to give up 1 CD to produce 1 DVD 
(100/100) 100 divided by 100 
But country B has to give up just 0.25 CDs to produce 1 DVD hence, country B is enjoying a comparative advantage in the production of DVDs