Answer:
In effect, the best answer to the question: The part "swaying in the wind" is:____, for the entire sentence: A small branch of a tree swaying in the wind brushed against the window, would be, 3) a verbal modified by a prepositional phrase.
Explanation:
The reason for this being the best answer is that sway, is a verb that is describing the action of the tree branch. This action, is also being modified directly by the phrase "in the wind", which is a prepositional phrase. It is a prepositional phrase because it possesses a preposition "in", followed by a noun, which is "the wind". So, "Swaying in the wind" is both a verbal that is being modified by a prepositional phrase and that is why the answer is the 3rd one.
