(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of the Assertion (A).
The textbook (Ch. 8) explicitly states that bacteria able to withstand heat survive better in a heat wave — confirming A. It also states that accumulation of variations gives species better chances of survival in a changed environment — confirming R. R directly explains A because heat-resistant bacteria survive due to the presence of accumulated variations in the population. Option (A) is therefore correct.