Final answer:
Immunoprecipitation is most efficient in the equivalence zone, where antigen and antibody ratios are approximately equal, facilitating maximal precipitation.
Step-by-step explanation:
Immunoprecipitation occurs when the ratio of antigen and antibody is optimal, allowing for the formation of a precipitin or lattice. The correct answer to your question is when the antigen and antibody ratios are approximately equal, that is, when the reaction enters the equivalence zone, where both the optimal antigen-antibody interaction and maximal precipitation occur. This corresponds to answer choice B) antigen and antibody ratios are approximately equal. Beyond this point, if more antigen continues to be added, the ratio will shift to an excess of antigen, resulting in the decline of precipitin formation.