By 2025, the most appropriate choice is B. "will be living."

Here's the explanation:

- A. "will live" suggests a future action but doesn't indicate ongoing action, which doesn't fit well when talking about a gradual and continuous process like establishing a human presence on Mars.
- B. "will be living" indicates an ongoing action in the future, which is appropriate given that establishing a human presence on Mars would be a continuous effort at that point in time.
- C. "will have lived" suggests that by 2025, humans would have completed their stay on Mars, which is highly unlikely given the current progress and timeline of space exploration.
- D. "will have been living" suggests a continuous action that began in the past and continues up to a point in the future, implying humans have already been living on Mars for some time before 2025, which again is unlikely at this point.

Given the ambitious nature of space exploration and the various challenges involved in establishing a permanent or semi-permanent human presence on Mars, option B most accurately represents the ongoing nature of such an endeavor by 2025.