Correct answer is (d) antibody
The explanation is: Antibodies are Y-shaped protein molecules manufactured by the body that help fight against foreign substances called antigens. These are also called immunoglobulins. Antigen is a substance that stimulates the immune system to produce antibodies. Antigen covalently linked to the enzyme is referred to as antigen enzyme conjugate, whereas and antibody covalently linked to enzyme is referred to as antibody enzyme conjugate.