The passage focuses on the diversity of asteroids and comets, as shown by the dedicated chapters and review questions discussing these celestial objects within the solar system. The correct answer is option A.
The primary focus of the passage is indeed best encapsulated by option A: "The diversity of asteroids and comets." The outlined chapters, ranging from 13.1 to 13.4, delve into intricate details regarding asteroids and comets, exploring their composition, behaviors, and significance within our solar system.
The mention of asteroids in the context of planetary defense and the examination of the nature and origin of comets further solidify the centrality of these celestial bodies in the discussed material. While planets and other cosmic entities are acknowledged, the specific chapters, coupled with the emphasis on asteroids and comets, align with the notion of exploring the diversity and characteristics of these celestial objects.
Therefore, Option A is correct.