Flying is the present participle in the sentence.
Present participle is the form of a verb, ending in -ing in English, which is used in forming continuous tenses, e.g., in I'm thinking, alone in nonfinite clauses, e.g., in sitting here, I haven't a care in the world, as a noun, e.g., in good thinking, and as an adjective, e.g., in running water.
The correct answer between all the choices given is the third choice or letter C. I am hoping that this answer has satisfied your query and it will be able to help you in your endeavor, and if you would like, feel free to ask another question.