ANS:B - Antigens and antibodies are in optimal proportion

In equivalent zone, ab and ags are present in optimum conc, as abs are usually divalent and ags are multivalent the cab form bridge between 2 or more ags this linking result in a formation of the multimolecular lattice which makes the reaction visible to the eye.

In the case of post zone and prozone phenomenon, there is no proper linking occur and is less visible to the eye.