The correct option is
B Antibodies
Anti-serum contains antibodies, such as that obtained from an animal that has been exposed to an antigen (e.g., pathogens). So, the serum that the animal makes contains the antibodies against antigen. Now, this serum containing the antibodies is used in passive immunity and is called as the anti-serum (since it works against the antigen).Therefore, the correct answer is option B.