Well, since grinning from ear to ear is an idiom, and that no one can do that, that will be your answer (I actually did this on my test, so I know the answer. A is wrong, because it just says is my, C is wrong, because there are boys, but it is not a participal phrase, and D is just talking about the boy's characteristics.
So your answer is B. grinning from ear to ear.
Hope this helped!
Nate