Which of the following organic manures has the lowest C/N ratio?

Answer: Option C

Organic manures that have the lowest C/N ratio are Groundnut cake from all the given options whereas compost, FYM, and Biogas slurry are highly rich in C/N ratio.
However, the C/N ratio of any substance is the amount of carbon about the amount of nitrogen present.
Groundnut cake is the residue of oil extraction from groundnut. The NPK ratio of this cake is 7.3 : 1.5 : 1.3 respectively. Its C/N ratio is less in comparison to the other options provided Although it helps in maintaining the C : N ratio in the soil.