Final answer:
The most effective replacement for sentence 6, given the context provided, is option C, which emphasizes the shared experience of all life on Earth and fits well with the overall theme of the transformative impact of space exploration on human perspectives.
Step-by-step explanation:
To provide a more effective thesis statement that best captures the main idea of the discussion regarding human perspective following the study of Earth from space and in the context of the vastness of the universe, option C would be the most appropriate replacement for sentence 6. The statement "After seeing Earth from space, they realize that all living beings on Earth share the same planet." underscores the unity and shared experience of all life on Earth, aligning well with the concepts of humility and change in perspective caused by astronomical discoveries mentioned in the provided text references. It echoes the sentiment of overcoming 'Earth chauvinism' and reframes humanity's place within a much larger cosmic context, as well as the transformative impact of viewing Earth from space.