I'm pretty sure it's a because efc means that the point of one of the ends of the angle will be part of the end of angle bfe meaning it can be alternate and seeing as how this is the only angle which either starts or ends with the letters b or e it has to be a if you can follow?

That was what I was thinking but now I'm quite confused however I think it has to be c or d now because for it to be alternate and exterior it couldn't have any of the same letters...I think