1. The Logic Structure :
- The Pivot: The word "Despite" creates a contrast between how the Mongolian Plateau or Altai Plateau looks ("rugged/tough") and what it actually is (the blank).
- The Clue: The second half of the sentence (after the colon) explains the blank. It states that even "fleeting" (quick/minor) human presence leaves a "lasting mark" or "deleterious [harmful] impact."
- The Inference: If a tiny bit of pressure causes long-lasting damage, the Mongolian Plateau or Altai Plateau is easily hurt or delicate, regardless of its "tough" look.
- Selection: Vulnerable (D). This is the only word that captures the idea of being easily damaged or susceptible to harm.

2. Evaluating the Choices :
- A. formidable: (Thematic Trap) Means inspiring fear or respect through being impressively large or powerful. This matches the desert's "rugged appearance," but the sentence is looking for the contrast to that appearance.
- B. resilient: (Opposite Trap) Means able to recoil or spring back into shape after bending or being compressed; able to withstand or recover quickly from difficult conditions. This is the exact opposite of what the clue (lasting damage from small steps) describes.
- C. precarious: (Category Trap) Means not securely held or in position; dangerously likely to fall or collapse. While a desert ecosystem might be "unstable," vulnerable is a much more precise fit for the specific clue of "damage from footsteps."
- D. vulnerable: (Correct) Susceptible to physical or emotional attack or harm. In an ecological context, it describes a system that cannot easily recover from disruption.
- E. forbidding: (Thematic Trap) Means unfriendly or threatening in appearance. Like formidable, this describes the "rugged" look rather than the "easily damaged" reality.