I would definitely say B.) A smile because if you were just staring it may be rude or distracting to the speaker and a smile shows you are listening and absorbing the knowledge
There’s no context but I think B) the sonnet
It gives the reader an understanding of how a character understands their world. My favorite example of this was in 'The Boy in the Striped Pajamas'. While the main character isn't the narrator, it gives the reader insight into how much the character did NOT understand of his world.
