Answer:
The participial phrase is "crunching popcorn".
Step-by-step explanation:
A participial phrase is a phrase that looks like a verb, but actually functions as an adjective; it modifies a noun in the same sentence. The phrase "Crunching popcorn" tells us about the "teenage girl". Also, the participial phrase at he beginning of a sentence should always be set apart by a comma from the rest of the sentence.