It would be answer "a"- because if you are not putting the commas in, it would be just a run-on sentence. So, it would be, "World War II, which lasted from 1939 to 1945, was a global conflict involving nearly every nation in the world." Having the commas in will help the reader digest and sink the information in on what the topic is, without the sentence just running through without any commas. Hope this helps. =)
