The correct option is C 1.Slurry 2.Digester 3.Biogas holder 4.Spent slurry
Biogas plants are simple structures. The tank which has the slurry has a inlet pipe through which the slurry enters the digester which is 10 to 15 feet deep concrete tank. Within the digester, the microbial action of the methanogens in the dung, slowly converts this dung-slurry into waste sludge releasing methane and hydrogen gas. The waste sludge accumulates in an outlet chamber on the side of the digester, as spent slurry, while the gas is captured by a floating cover placed on top of the digester, called the biogas holder. A pipe on the top of the cover allows gas to be withdrawn and used within the house as needed. Even the waste/spent slurry is useful as it has a lot of nutrients and can be used as a biofertilizer.