Final answer:
Soluble antigens are detected in a precipitation reaction, where soluble antigens added to a solution containing antibodies form a visible precipitin complex.
Step-by-step explanation:
This involves adding soluble antigens to a test tube containing a solution of antibodies to form a precipitin. A precipitin reaction typically involves polyclonal antibodies because they can bind to multiple epitopes, making lattice formation more likely and a visible precipitin will form. This indicates that the correct answer is c. Precipitation. In contrast, agglutination assays involve antibodies aggregating cells or large particles into a visible matrix and can be used to detect the presence of specific antigens or antibodies in patient serum.