In a predator-prey coevolution, if the prey gains a defense to stop predation, then the predator may evolve..?

Its a. Its called the Red Queen affect. If the prey species develops a new defence mechanism, the predators find a new way around it. Plus any new defence will occur gradually, so its easy for the predators to develop a new way to counteract that over time.

a of course. There are many examples of co-evolution analogous to an arms race. As Dawkins wrote, much of what is observed in the natural world can best be explained as the "result of long and bitter arms races." That is why some snake venom is extremely potent; some prey evolved a tolerance to weaker venom, necessitating the evolution of stronger venom. That's why some predators like eagles have very keen eyesight, to spot prey which lies motionless and camouflaged. That's why Tyrannosaurs evolved large size and powerful jaws and teeth, to overcome the big, tough titanosaurs, ceratopsids, ankylosaurs etc of its environment.