Right choice is (d) De-acidification
The explanation is: De-acidification is not carried out in anaerobic digestion. Anaerobic digestion is carried out by various bacteria involved in three major steps of anaerobic digestion. The entire process is divided into three steps that is hydrolysis, acidification and methane formation or methanogenesis. Hydrolysis converts organic matter; Acidification forms acetic acid and methanogenesis leads to methane.