Classification — select the set where all three are measurement units (standard or commonly used).

Introduction / Context:
This classification problem asks you to identify the only set in which all items are measurement units. Recognizing domain labels (devices, animal terms, money, or physical units) quickly filters candidates.

Given Data / Assumptions:

• Accept common measurement units (e.g., hertz for frequency, watt for power, calorie for energy). Minor spelling variants are normalized (e.g., “Calorie”).
• Exactly one option must contain three measurement units; mixed or category labels (like “Currency”) do not qualify.

Concept / Approach:
Label each item by domain and reject any option that has a non-unit member. Domain cues: “Keyboard/Mouse” are computer peripherals; “Leveret/cub/Elephant” are animal terms; “Yen/Pound/Currency” mixes two monetary units with a category name rather than a unit; “Hertz/Calorie/Watt” are all units.

Step-by-Step Solution:
1) A: Keyboard (device), Mouse (device), Category (abstract label) ⇒ not units ⇒ reject.2) B: Leveret (young hare), cub (young mammal), Elephant (adult animal) ⇒ not units ⇒ reject.3) C: Yen (currency unit), Pound (ambiguous: currency or mass unit, but here the word “Currency” is a category label, not a unit itself) ⇒ mixed/ill-posed ⇒ reject.4) D: Hertz (frequency), Calorie (energy), Watt (power) ⇒ all measurement units ⇒ accept.

Verification / Alternative check:
Ask: could each be followed by a numerical magnitude and physical quantity (e.g., 60 hertz, 500 calories, 100 watts)? For D yes; for A–C, not consistently.

Why Other Options Are Wrong:
They contain peripherals, animal terms, or an abstract category label rather than exclusively units.

Common Pitfalls:
Seeing “Pound” in C and assuming the entire set qualifies; however, “Currency” is not a unit, and the set is not uniformly measurement units. Consistency across all three items is required.

Final Answer:
Hertz, Calorie, Watt