Assertion-Reason check method: First test Assertion (A) alone, organic farming is widely documented to promote environmental sustainability through improved soil health, biodiversity and reduced pollution, so (A) is true.
Next test Reason (R) alone, organic farming indeed avoids synthetic pesticides and fertilizers, and this avoidance is what preserves soil health, conserves water quality and reduces air/water pollution, so (R) is true.
Finally check the logical link, does avoiding synthetic chemicals (R) explain why sustainability is promoted (A)? Yes, the mechanism described in R is precisely the cause of the outcome stated in A.
Since both statements are independently true and R logically explains A, the first option is correct.