SONAR, used to find the depth of the sea and to locate submarines, works on the principle of:

The correct answer is B, reflection of ultrasonic waves. SONAR stands for Sound Navigation and Ranging. A transmitter on the ship sends short ultrasonic pulses into the water, the pulses are reflected from the sea bed, a submarine, an iceberg or a shoal of fish, and a detector records the returning echo. Since the pulse travels down and back, the depth equals the speed of sound in water multiplied by the time taken and divided by two.

Option A is wrong because light is absorbed within a short distance in sea water and is useless for sounding the deep ocean. Option C is wrong for a similar reason, as radio waves are strongly attenuated in salt water, which is why submarines must surface or use very low frequencies to communicate. Option D names the wrong band and the wrong phenomenon: infrasound is below the audible range and interference is not how ranging is done.