Answer: B. They signal phagocytes
Explanation: Complement proteins are a part of immune system of the body. These are plasma proteins which help in the process of marking and killing the pathogens. These are not involved in the fever causing mechanism, so option D is incorrect. Also, these are not carrier proteins, and are not associated with hormones, thus option C is also incorrect. Since these proteins bind with the pathogens and make them easily recognized by the phagocytes, option B is the right answer.