The participle in the sentence is soaring (c). A participle is formed from a verb to serve as an adjective. Therefore, the hawk is "soaring" which is from the verb soar which makes it the participle in the sentence.
The words 'soaring' and 'spotted' in the sentence are both participles. 'Soaring' describes the hawk's action, while 'spotted' indicates what the hawk did. Your choice of 'soaring' (C) is correct, but 'spotted' (D) is also a participle.
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