Analogy — Map substance to its Earth system sphere: Air : Atmosphere :: Water : ?

Introduction / Context:
Earth system science groups components into “spheres.” Air belongs to the atmosphere. We must choose the analogous sphere for water.

Given Data / Assumptions:

• Atmosphere → air and gases.
• Hydrosphere → water in all forms (oceans, rivers, lakes, ice as part of the cryosphere subset).
• Biosphere → living organisms; stratosphere is one layer of the atmosphere; ecosphere may be used broadly for the total ecosystem but is not the standard sphere for water.

Concept / Approach:
Preserve “material → principal Earth sphere.”

Step-by-Step Solution:
Air → atmosphere.Water → ? → hydrosphere.

Verification / Alternative check:
Standard classification: lithosphere (rock), hydrosphere (water), atmosphere (air), biosphere (life).

Why Other Options Are Wrong:
(b) Ecosphere – broad ecological concept; not specific to water.
(c) Biosphere – life, not water.
(d) Stratosphere – a subset of atmosphere; still about air/gases.

Common Pitfalls:
Choosing a term that sounds “eco-related” rather than the scientifically precise sphere.

Final Answer:
Hydrosphere