The answer is: [A]: atmosphere .
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Explanation: Lightning does occurs in the "atmosphere". (Think: clouds/weather).
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There is no such thing as the "magnetosphere", in terms of the "4 (FOUR) spheres---geosphere, biosphere, atmosphere, hydrosphere.
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Lightning would not be found in the "biosphere"; lightning is not an "organic" process; nor does it directly occur in or within the ecosystem or living environment.
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Lightning, in and of itself, it independent of the "hydrosphere"/ hydrologic cycle/hydrologic processes that occur on Earth.
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