Answer:
Option A, As individuals evolve to better fit their environment, their environment also changes, subjecting the individuals to continually changing selective pressures.
Step-by-step explanation:
By running the author means that an organism has to continuously evolve in order to cope up with the changing environmental conditions and hence survive and be fertile.
As per the theory of natural selection, an individual that is best fit to the prevailing environmental condition is able to survive and reproduce. And in order to be fit, an organism is required to evolve continuously.
Hence, option A is correct