Correct option is A
A Fathometer is an instrument used to measure the depth of water, particularly in oceans and seas. It works on the principle of sonar, emitting sound waves that travel to the sea floor and reflect back to the instrument. The time taken for the sound waves to return is used to calculate the depth of the water. The term "fathom" is a unit of length used in reference to the depth of water, where one fathom equals six feet.