ELISA is based on the principle of:

The correct option is Antigen - antibody interaction.

ELISA stands for Enzyme-Linked Immunosorbent Assay. It is a widely used laboratory technique designed to detect and quantify substances such as peptides, proteins, antibodies, and hormones in a biological sample.
The fundamental principle behind ELISA is the highly specific binding between an antigen and its corresponding antibody.

In this assay, one of the components (either the antigen or the antibody) is immobilized on a solid surface, typically a multi-well microtiter plate. When the sample is added, the specific complementary partner binds to the immobilized molecule. This interaction is then detected using an enzyme-linked conjugate that produces a measurable signal (usually a color change) upon the addition of a specific substrate. Because the assay relies directly on this biochemical recognition, the correct principle is the Antigen - antibody interaction.