Ultrasound Defined by Frequency Above 20 kHz Steps:

• Ultrasound is sound with frequency f > 20,000 Hz (upper human hearing limit).
• Calculate f = v / λ for each option, with v = 330 m/s; convert λ to meters.
• Option A: λ = 0.25 m, f = 330 / 0.25 = 1,320 Hz (audible).
• Option B: λ = 0.025 m, f = 330 / 0.025 = 13,200 Hz (audible).
• Option C: λ = 0.0025 m, f = 330 / 0.0025 = 132,000 Hz (ultrasound).

Why C is correct:

• Its frequency of 132,000 Hz exceeds 20 kHz, meeting the ultrasound definition.

Why the others are wrong:

• A: 1,320 Hz is low audible sound.
• B: 13,200 Hz is high audible sound.
• D: Incorrect, as C is ultrasound.

Final answer: C