Answer: The bolded word in the given sentence is "hilly" and it is an object complement.
Step-by-step explanation: An object complement is a word or a group of words (nouns, pronouns or adjectives) that follows a direct object in a sentence, and it renames it, describes it or states what it has become. In the given sentence, the direct object is "land" and the word "hilly" is placed after the direct object and it states what it has become (the flat land has become hilly), so the correct answer is option A.