In a double immunodiffusion (Ouchterlony) test, what is the ‘zone of equivalence’?

A zone created by low ionic strength in the gel that prevents diffusion, causing antibody to remain concentrated in the well.

The area near the antigen well where very high antigen concentrations produce a diffuse cloud of precipitate rather than a discrete line.

The central point of the plate where diffusion stops and no interaction can occur because antibody has been completely consumed.

The region in the gel where antigen and antibody concentrations are optimal and balanced such that a visible precipitin line forms due to lattice formation.