The increase in crop yield with green manure is approximately
(a) 1-$10\%$
(b) 10-$20\%$
(c) 30-$50\%$
(d) 60-$80\%$

Hint: Manure is a type of organic matter that is used in agriculture as an organic fertilizer.
The plants that are used for green manure are ploughed under or harvested from the soil as they are green or have completed flowering.

Complete Answer:
Agricultural manure mostly consists of animal feces. Other sources of manure includes
1. compost
2. Green manure
Manures increase the fertility of soil by adding nutrients and organic matter to the soil and in turn fertilizing the soil.
Agricultural green manure is manufactured by leaving uprooted and withering parts of plants on a piece of land to let them wither further. These uprooted and withering parts of plant parts serve the purpose of acting like mulch and soil conditioners. Green manure plays a very important role with respect to organic farming and it also plays an important role in the production of sustainable yearly cropping systems. The increase in crop yield with green manure is approximately 30-$50\%$.

Therefore, the correct answer is (c) 30-$50\%$.

Note:
The nutrients and organic matter produced by the manure are used by bacteria and other organisms like fungi in the soil. Organisms of a higher order consume these bacteria and fungi and maintain the food web of the soil.
The plants that are grown for the purpose of formation of green manure are solely grown as cover crops for the purpose of production of green manure.