Spent slurry (Bio-waste after obtaining bio-gas) is used as
(a) fuel (b) manure (c) food for livestock (d) used again for generating biogas

(b) manure

Explanation

In a biogas plant, animal waste like cow dung is used to make slurry which is made by mixing cow dung and water in equal proportions in the mixing tank. After obtaining the biogas, the spent slurry is used as manure.