Answer:
The adverb clause modifies the verb phrase "began whispering and goggling".
Step-by-step explanation:
An adverb clause is a group of words or clauses that modifies a verb, an adjective, or an adverb in a sentence. This means that an adverb clause will change or qualify the verb, adverb, or adjective in a sentence.
The adverb clause in the given sentence is "once they saw the teacher close the book". And this adverb clause modifies the verb phrase "began whispering and giggling". The verb phrase contains the main verb and the words accompanying it, the verb dependents, thereby making it a phrase.
Thus, the adverb clause modifies the verb phrase.