In basic general knowledge of scientific instruments, a fathometer is an instrument used to measure which of the following?

Introduction / Context:
This general knowledge question asks about the use of a specific scientific instrument called a fathometer. Many competitive exams test familiarity with common instruments and what they measure, such as barometers for atmospheric pressure or thermometers for temperature. Knowing what a fathometer measures is particularly relevant to geography, navigation and basic science awareness.

Given Data / Assumptions:

• The instrument in question is a fathometer.
• The options suggest different physical quantities: earthquakes, rainfall, ocean depth and sound intensity.
• We need to recall which physical quantity is associated with the fathometer in standard terminology.
• No advanced physics is required; only factual recognition is needed.

Concept / Approach:
The word fathometer hints at the unit fathom, which is a traditional unit of length used especially for measuring water depth. A fathometer is an echo sounding device that uses sound waves to determine how deep the water is under a ship or at a particular location. The instrument sends pulses, measures the time until the echo returns from the seabed and converts this time to depth. Therefore, the correct association is with measuring depth in oceans, seas or other large water bodies.

Step-by-Step Solution:
Recall the meaning of the unit fathom. One fathom equals six feet, and it is historically used by sailors to describe water depth. Recognise that a fathometer is named after this unit and is designed to measure distance from the water surface to the bottom, i.e., depth of water. Check option C: “Ocean depth”. This directly matches the purpose of an echo sounding device used on ships and in hydrographic surveys. Check other options to ensure they describe different instruments. Earthquakes are measured by seismographs or seismometers, not fathometers. Rainfall is measured by a rain gauge, an instrument that collects precipitation over a given period. Sound intensity is measured using sound level meters or decibel meters, not fathometers.

Verification / Alternative check:
You can verify this by recalling context where you may have heard the term. Fathometers are commonly mentioned in connection with ships, submarines and nautical charts. They use the principle of sonar: sending sound waves downwards and recording the echo. The time taken for the echo to return is converted to a distance using the known speed of sound in water. This distance is exactly the water depth, which confirms that the fathometer is a depth measuring instrument, especially for oceans and seas.

Why Other Options Are Wrong:
Option A, earthquakes, is associated with seismographs, not fathometers. Option B, rainfall, is associated with rain gauges or pluviometers. Option D, sound intensity, refers to how loud a sound is and is usually measured in decibels using sound level meters. While a fathometer does use sound, it does not measure intensity but the time taken for sound to travel and return, which is then converted to depth.

Common Pitfalls:
Students may confuse instruments that involve sound. Because a fathometer uses sound waves, one might mistakenly link it to sound intensity. Another mistake is to guess earthquakes due to the presence of the word meter, thinking of seismometer. Remember that the prefix fathom strongly suggests depth measurement. Associating keywords like baro for pressure, thermo for temperature and fathom for depth can help keep these instruments clear in your memory.

Final Answer:
A fathometer is used to measure ocean depth, so option C is correct.