Answer :
D. soaring is adjective for eagle.
A. across is a preposition.
B. slowly is an adverb.
C. flying is a verb.
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The correct answer is D.
The word "soaring" is used as an adjective and is modifing the word "eagle," wich is the subject of the sentence.
A is not correct because "across" is a preposition.
B is wrong because "slowly" is an adverb modifying a ver.
C is also incorrect either because "flying" is the main verb of the predicate.