This passage is about a time when

The question asks to identify the time period a passage is about, based on the provided options. Although the full passage is not visible, we can analyze the options in the context of historical periods related to European empires.

• A. most of the empires had been formed to a large extent. This suggests a later stage of imperial development, where the major empires are already well-established.
• B. virtually all the colonies had been established. This implies an even later stage, nearing the peak or end of colonial expansion.
• C. Europeans knew nothing of the world outside Europe. This describes a period before the Age of Exploration, when significant European empire-building overseas had not yet begun.
• D. European powers were about carving out their empires. This describes the active period of exploration, conquest, and initial colonization, where European nations were competing to establish and expand their overseas territories and influence. This phase precedes the full formation or establishment of most empires and colonies.

Given these options, if a passage is about the process of empire formation, option D best describes the dynamic period of initial expansion and competition.

The final answer is $\boxed{\text{D}}$