The main topic of the passage is the diversity of asteroids and comets, as indicated by the outlined chapters and summary focusing on asteroids and comets within the solar system. The correct answer is option A.
The main topic of the passage is A. The diversity of asteroids and comets. This is evident from the chapter outline and summary provided, which focus on the various types of asteroids, the concept of planetary defense against asteroids, and the nature and origin of comets.
Additionally, the passage discusses the role of both asteroids and comets in contributing to our understanding of the solar system's history, highlighting their significance as 'debris of the solar system.' The content does touch upon celestial bodies and the solar system at large, making option C a general theme, but the specificity of the chapters points more directly towards the diversity of asteroids and comets as the main topic.
Therefore, option A is correct.