In the sentence, forebodingly is used to create suspense. By saying "The ship creaked forebodingly", the narrator is hinting at some impending conflict or doom. While the words uneasiness, alert, and apprehensive can also create the same type of mood, they are not things that can describe how the ship creaked. An adverb must be used. Ominously is the only adverb that is listed and it creates the same feeling of suspense and impending conflict.
