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
The participial phrase is option B) “grinning from ear to ear". A participial phrase is a set of words form by the participle (verb+ing) and always acts as an adjective. In this example, the participial phrase “Grinning from ear to ear” is modifying the noun “boy”.
The rest of the options are not correct since they are not formed by a participle. For example, option 4) “mischievous little brother” included the adjectives “mischievous little” that are modifying the noun “brother” but the adjectives are not form by a participle (verb+ing).
