The frequencies of four sound waves are given below. Which of these sound waves can be used to measure the depth of sea by the echo method?
(a) 15,000 Hz
(b) 10 kHz
(c) 50 kHz
(d) 10,000 Hz

The ultrasonic waves of $50000\ Hz$ or $50\ kHz$ high frequency can be used to  measure the depth of sea by the echo method.

So, option (c) is correct.