A parasite depends on its host to stay alive. Killing the host before the reproduction destroys parasite also as it needs host to produce its progeny generation after generation. This makes option D correct. In order to keep the host alive to produce many generations of parasite offspring, it is necessary for a parasite to become less virulent or gentle over many generations. This makes option A incorrect. Fast reproduction produces increase the number of parasite offspring but poses extra burden on host and makes the host survival harder. This makes option B incorrect. Similarly, sticking too long to a host decreases its survival chances and hence, parasite keeps switching the host over time. Option C is incorrect. Thus, the correct answer is option D.