Lignin theory is given by

Answer: Option A

Lignin's theory is given by Waksman in 1932. According to this theory, lignin is incompletely utilized by microorganisms and the residuum becomes part of the soil humus.
Modification in lignin includes loss of methoxyl groups with the generation of o-hydroxy phenols and oxidation of aliphatic side chains to form COOH groups. The modified material yield first humic acids and then fulvic acids. This pathway is exemplified by Waksman’s lignin-protein theory.