The correct answer is (C) against. The phrasal verb "take against someone" means to develop a sudden dislike or prejudice towards that person. The context of the sentence, "But I'm not worried; I can take care of myself," strongly supports this interpretation, indicating that the speaker perceives a negative sentiment or hostility from the other person. This idiom perfectly captures the idea of someone forming an immediate aversion.
Let's consider the distractors: "take off" typically means to depart, remove clothing, or achieve success, none of which fit the sentence's meaning of developing a feeling towards someone. "Take over" means to assume control or responsibility, which is also irrelevant to the context of personal interaction and immediate dislike. Therefore, "against" is the only preposition that forms a coherent and contextually appropriate phrasal verb, conveying the intended meaning of developing an instant negative feeling towards someone.