Classification – Scientific instruments vs unrelated term: select the odd one out. Three are instruments used to measure specific physical properties; one is not an instrument but a geometric term. Which option does not belong? Options: Hydrometer, Barometer, Diameter, Hygrometer.

Introduction / Context:
Scientific vocabulary often clusters around instruments and what they measure. Hydrometer, barometer, and hygrometer are measuring devices; diameter is a length attribute in geometry, not an instrument.

Given Data / Assumptions:

• Hydrometer: measures relative density/specific gravity of liquids.
• Barometer: measures atmospheric pressure.
• Hygrometer: measures humidity.
• Diameter: a geometric length across a circle/sphere, not a device.

Concept / Approach:
Group by “instrument” vs “non-instrument term.” The non-instrument is the odd item.

Step-by-Step Solution:
Step 1: Mark which options are measuring devices.Step 2: Hydrometer/Barometer/Hygrometer are instruments.Step 3: Diameter is not an instrument; it is the exception.

Verification / Alternative check:
Suffix “-meter” often indicates an instrument; “diameter” describes a metric, not the device.

Why Other Options Are Wrong:
Hydrometer/Barometer/Hygrometer: All are valid instruments.

Common Pitfalls:
Beware of spelling variants (e.g., “hygrometer” vs “hygrometre”); the function remains measurement.

Final Answer:
Diameter