Correct option is C
The concept of critical soil test level was proposed by Cate and Nelson in 1971. According to their concept, the critical soil test level is the nutrient concentration in the soil above which there is no significant increase in crop yield with additional nutrient application.
It helps in determining the nutrient sufficiency limit for optimum crop production.
Fertilizer application beyond the critical level does not significantly enhance yield.
Muhr: Muhr was involved in developing fertilizer recommendations and soil testing methods in India.
Gregory: Gregory contributed to plant breeding and agronomy.
Mitscherlich: Mitscherlich proposed the Law of Diminishing Returns related to nutrient uptake and yield.