Concept:The biosphere is the global zone where living organisms exist and thrive.
Explanation:The biosphere includes parts of the atmosphere, hydrosphere, and lithosphere where life is found.
It is the only zone that supports plants, animals, and microorganisms together.
The atmosphere, lithosphere, and barysphere individually are not life-supporting in the same complete sense.
Therefore, the part of Earth's external structure that supports life is the biosphere.
Answer:A. biosphere