Water : Sand :: Ocean : ? — Preserve the “vast collection of …” relation between natural domains and select the correct counterpart.

Introduction / Context:
“Water : Sand” evokes the idea of primary substance associated with a natural domain composed largely of that substance. We must replicate the same domain–substance relation for “Ocean.”

Given Data / Assumptions:

• A desert is predominantly a vast area of sand (and rock), often conceptualized as “sand domain.”
• An ocean is a vast body of water.
• The analogy appears to match domain-by-constitutive medium.

Concept / Approach:
Map “Water : Sand” as “domain medium A : domain medium B.” Then, pair ocean with the domain analog that is to sand as ocean is to water: “Ocean : Desert.” This preserves the correspondence of large natural expanse characterized chiefly by a single material (water ↔ sand).

Step-by-Step Solution:
1) Recognize composition relation: ocean → water, desert → sand.2) The first pair lists the mediums; the second pair should list the domains.3) Therefore, Ocean : Desert matches Water : Sand.

Verification / Alternative check:
Alternate proposals like “Ocean : Waves” fail because waves are phenomena within the ocean, not the analogous “domain” counter to a desert. “River” is a water body but not the counterpart to “desert” in a medium-composition sense.

Why Other Options Are Wrong:

• Engine: Irrelevant.
• River: A water body; fails the cross-domain sand analogy.
• Waves: Processes/features of oceans, not domain analogues.
• Shore: Boundary region, not a domain primarily characterized by a medium.

Common Pitfalls:
Matching ocean with a water-related term (river, waves) instead of seeking the sand-dominated domain that mirrors the relationship structurally.

Final Answer:
Desert