Antiserum is

The correct option is blood serum containing specific antibodies.

To understand why this is correct, let us break down what antiserum is:
1. Blood Serum: When blood clots, the liquid portion that remains after the clotting factors and blood cells (like red and white blood cells) are removed is called serum.
2. Antibodies: These are protective proteins produced by the immune system in response to foreign substances (antigens) like bacteria, viruses, or toxins.
3. Antiserum: Antiserum is blood serum obtained from an animal or human that has been immunized or has recovered from an infectious disease. Because of this exposure, the serum contains high concentrations of specific antibodies that can bind to and neutralize a particular antigen.
Therefore, antiserum is defined as blood serum containing specific antibodies, which can be used to provide passive immunity or to treat specific infections and toxic bites (like snake venom).