Answer:
D: adverb phrase
Step-by-step explanation:
The phrase in silent agony is an adverb phrase because it is describing how the sick whale died. The adverb is modifying the verb died. The function of adverbs is to modify verbs, adjectives or other adverbs. Many adverbs end in ly; in this sentence we can take the words in silent and see that they are equivalent to silently.