The correct option is C Peroxidase
ELISA is 'Enzyme Linked Immunosorbent Assay'. It is a commonly used diagnostic test to detect antibodies in the blood. In this test, the antibody is used as the substrate. The enzyme used in this diagnostic test is horseradish peroxidase. In the presence of specific substrates, which act as hydrogen donors, this enzyme converts colorless or non-fluorescent molecules into colored and/or fluorescent moieties respectively due to its reduction.
Thus, the correct answer is option A.