Answers:
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[A]: lithosphere (more commonly—and relatively more recently— known as the "geosphere" ) ;
to avoid confusion with the other definition of the "lithosphere" — however, in this context, this answer is correct).
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[B]: "biosphere" .
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[C]: "hydrosphere" .
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Note: The remaining one that is not among the answer choices given is the: "atmosphere" .
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Of course, there are other "separate subsystems" that are not among the "4 MAJOR realms; such as ionosphere, heliosphere, cryosphere.
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