In S.I. units, the hertz (Hz) quantifies how many cycles occur per second. Identify the physical quantity measured in hertz.

Introduction / Context:
The hertz (Hz) appears across many fields: alternating current systems, vibrations, acoustics, and wave phenomena. Correctly associating Hz with the underlying quantity ensures clear interpretation of time-dependent processes in engineering and physics.

Given Data / Assumptions:

• Unit under consideration: hertz (Hz).
• By definition, 1 Hz = 1 cycle per second.
• Other options list different base or derived quantities with distinct units (A, W, cd, J).

Concept / Approach:

Frequency counts how often a periodic event repeats per unit time. This aligns precisely with cycles per second, the definition of hertz. Luminous intensity (cd), electric current (A), power (W), and energy (J) are unrelated quantities with different units.

Step-by-Step Solution:

Verification / Alternative check:

AC mains examples: 50 Hz or 60 Hz indicate 50 or 60 complete waveform cycles per second—textbook illustrations of frequency.

Why Other Options Are Wrong:

• Luminous intensity: candela (cd).
• Electric current: ampere (A).
• Power: watt (W).
• Energy: joule (J).

Common Pitfalls:

• Confusing angular frequency (rad/s) with cyclic frequency (Hz). Relation: omega = 2pif.
• Interpreting 'Hz' as amplitude rather than rate of repetition.

Final Answer:

frequency.