I believe that Robinson teaches Poll to Speak B. to ease his loneliness.
He is alone, stranded on an island, and has no one to talk to. So in the end he teaches his parrot, Poll, how to speak in order to hear another being's voice. It's not to occupy time, because he has plenty to do on the island, like planting seeds, building a home, etc. There are no enemies to scare off, so C is also incorrect. D is incorrect because he writes down days as they go by.
