The small gene pool of cheetahs today can be attributed to several factors, but the most significant is D) Multiple bottleneck events.
Cheetahs have gone through several population bottlenecks in their history, which significantly reduced their genetic diversity. These events were caused by factors like habitat loss, climate change, and hunting pressures, leading to a limited number of individuals that contributed to the current population gene pool. While factors A, B, and C (poor adaptability, lack of advantageous traits, and inbreeding in zoos) can also play a role in the issues surrounding cheetah genetics, they stem largely from the historical bottlenecks that drastically reduced population size and genetic variation.