A. MacArthur discusses change and the expanding reach of humanity, emphasizing that the cadets' duty as soldiers will remain constant. B. MacArthur addresses change and humanity's expanding reach, setting the stage for how the cadets' roles as soldiers will evolve. C. MacArthur speaks of exploring the universe and making exciting discoveries, which he believes he may not witness but expects the cadets to do so. D. MacArthur describes humanity's growing comprehension of the universe's vastness, offering the cadets insight into their modest role within it.

Options A, B, C, and D all present different summaries or interpretations of a hypothetical speech made by MacArthur to a group of cadets. Here's a breakdown of what each option suggests:

A. This summary implies that MacArthur discussed the broader context of change in the world and humanity's increasing influence. However, despite these changes he emphasizes that the core duty and function of the cadets as soldiers will remain unchanged.

B. This summary also mentions MacArthur talking about change and humanity's expanding role but suggests that these changes will alter the cadets' roles as soldiers implying that the cadets will need to adapt their duties in response to the changes.

C. The focus here is on MacArthur discussing the potential for exploration and discovery in the universe recognizing that although he might not live to see these advancements he expects the cadets will have a hand in them.

D. In this interpretation MacArthur is talking about humanity's increased understanding of the universe and the vastness of space giving the cadets perspective on how modest their role might be in the grander scheme.

Without additional context, it is difficult to determine which of these options if any accurately reflects what MacArthur said or which event is being summarized. However these summaries can serve as an overview of varying perspectives on a speaker's themes about duty change and humanity's future.