Observe the diagram and answer the following question.

What is indicated 'R' in the diagram?

Antigen binding site

The correct answer is Option (2) – Antigen binding site

When the V_H and V_L domains are paired in the antibody molecule, the hypervariable loops from each domain are brought together, creating a single hypervariable site at the tip of each arm of the molecule. This is the binding site for antigen, the antigen-binding site or antibody combining site.