Fill in the blank with the most appropriate word.
A preposition is a word which comes before a noun or a pronoun and establishes a relationship between the elements of a clause or words. Option D)of is correct as it indicates a characteristic or a reason (here flying). The other options are wrong as at, in, and of indicate a specific location, inside something, and possession respectively. These uses don't fit in the given question. The correct answer is D)of.