Answer:
C) in the Pocono Mountains
Step-by-step explanation:
Adverb phrases are phrases that modify, describe or gives further information of adjectives, verbs or other adverbs. Very often, we can identify them by answering the following questions: "Why?", "How?", "When?", "Where?" or "In what circumstances?".
In the sentence, C is the best option because it modifies the verb "worked" and it answers to the Where question: Where did you work? At the summer camp in the Pocono Mountains.