Fundamental frequency unit in SI: The unit of frequency is:

Introduction / Context:
Frequency—the number of cycles or events per second—is central in signal processing, power systems, and communications. Correct unit usage is essential for interpreting specifications like sampling rates, clock speeds, and mains frequency.

Given Data / Assumptions:

• Frequency f measures cycles per second.
• 1 hertz (Hz) = 1 s^−1.
• We distinguish frequency from electrical current (A), power (W), and time (s).

Concept / Approach:
The SI derived unit for frequency is the hertz, defined as one event per second. While second appears in the definition, second itself is a base unit of time; watt and ampere are unrelated quantities representing power and electric current respectively. Therefore, hertz uniquely identifies frequency.

Step-by-Step Solution:

Verification / Alternative check:
Check common values: mains frequency 50/60 Hz, audio range up to 20 kHz, RF signals in MHz/GHz—all stated in hertz multiples, confirming the standard.

Why Other Options Are Wrong:

• Ampere: Unit of current, not frequency.
• Watt: Unit of power, not frequency.
• Second: Base time unit; its inverse defines frequency but is not itself the frequency unit name.

Common Pitfalls:
Confusing angular frequency (rad/s) with frequency in Hz; remember ω = 2 * π * f.

Final Answer:
hertz