The poem isn't talking about one path being good versus the other path being bad. This can be seen when he says: "And both that morning equally lay." This means that the paths are equal and one isn't "higher" then the other. therefore the answer isn't A.
Choices ARE mandatory. Nothing in life would happen if we never had to decide anything. There fore the answer isn't C.
This leaves the answers B. and D. as the two key themes. As I said before choices are necessary (B.). D. can be seen when he says "I took the one less traveled by, And that has made all the difference." This means that he made a choice and making it has changed his life. Had he made another choice then his life would have been different.
