Verification of truth: A river necessarily has which of the following—choose the feature that must exist by definition.

Introduction / Context:
We must identify a defining feature of a river that is always present, regardless of geography, climate, or human use. This is a definition/essence check rather than a common-association check.

Given Data / Assumptions:

• Some features (delta, tributaries, boats) are conditional or human-dependent.
• We seek a feature inherent to any river's existence.

Concept / Approach:
A river is a natural flowing watercourse confined by two banks. Banks define and bound the channel; without banks there is no river channel, merely an unconfined body of water.

Step-by-Step Solution:

Verification / Alternative check:
Across headwaters, braided reaches, and meandering plains, the channel is always laterally bounded—i.e., has banks—even if shifting.

Why Other Options Are Wrong:

• Delta: not all rivers build deltas (e.g., strong tides/currents can prevent delta formation).
• Tributaries: some rivers have none.
• Boats: not intrinsic to a river.
• None of these: wrong because banks are necessary.

Common Pitfalls:
Equating “common” with “necessary” (boats, deltas are frequent but not universal).

Final Answer:
Banks