Basic Units – Identify the physical quantity measured in the nautical unit “fathom”.

Introduction / Context:
A fathom is a traditional nautical unit commonly used in maritime navigation and oceanography. Understanding which quantity it measures helps interpret nautical charts and marine reports, especially for safe anchoring and dredging operations.

Given Data / Assumptions:

• We must identify the physical quantity associated with “fathom”.
• Fathom is primarily used at sea.
• Typical equivalence: 1 fathom = 6 feet ≈ 1.8288 m.

Concept / Approach:
In marine contexts, fathoms are used to report water depth below the waterline or chart datum. While “distance” is a general term, the specialized application of “fathom” is specifically for vertical depth measurements.

Step-by-Step Solution:
Recall definition: fathom is a unit of length used for measuring depth of water.Associate domain: nautical charts mark soundings in fathoms.Select the option that states the specific quantity: depth.

Verification / Alternative check:
Cross-remember related units: nautical mile for horizontal sea distance; knot for speed; fathom for depth. This triangulation prevents confusion.

Why Other Options Are Wrong:

• sound: a phenomenon, not the measured quantity here.
• frequency: measured in hertz; unrelated.
• distance: overly broad; the specialized use is depth.

Common Pitfalls:
Conflating “distance” and “depth”. Although depth is a length, in maritime usage “fathom” almost always conveys vertical depth.

Final Answer:
depth