(C) (A) is correct but (R) is not correct.
Sustainable agriculture is indeed environment friendly, but it does not prevent use of chemical fertilizers — it involves avoiding/preventing adverse effects to soil, water and biodiversity by reducing chemical use, not an absolute prevention.
The Assertion is correct — sustainable agriculture uses environment friendly farming methods without damaging natural systems. However, the Reason is worded incorrectly; the textbook states sustainable agriculture "involves preventing the use of chemicals so as to avoid adverse effects to soil, water and biodiversity" — it aims to minimise harmful impacts, and the reason's phrasing ("prevents use of chemical fertilizers to protect soil") is an oversimplification/inaccuracy. Hence (R) as stated is not fully correct, making option (C) the right answer.