(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).
Microbes (bacteria and fungi) decompose dead organic matter, returning nutrients to the soil, thus enabling its natural replenishment.
The Reason directly explains why the Assertion is true — without microbial decomposition, dead matter would not break down and nutrients would not be returned to the soil, so replenishment would stop. Option (A) is correct because R is not just true but is the cause behind A. Remember: choose (B) only when R is true but explains something else, not the assertion itself.