"Two roads diverged in a yellow wood" is a metaphor for the choices and options that life offers us and the poet goes on to talk about such choices: how he took one road but wanted to take the other road also. However, the poet doubts, knowing "how way leads on to way", if he will ever get to come back and take the road. He says that some time in the future, he will be thinking of all this with a sigh. In this context, option B is the best answer to the question asked.