The depth of water is measured by using
A. Echo sounding
B. Diffraction
C. Refraction
D. Reflection

Hint: Think about the properties of sound waves like reflection. The echo sounder is used to measure the depth of water. They work on the principle of reflection of sound.

Complete step-by-step answer:
We can measure the height of a hill easily. When it comes to measuring the depth of the water in an ocean or a trench, we need some help. As you know the height of Mount Everest is 8848m or in terms of kilometers, it is about 8.848 km. Initially, it was challenging for the climbers to climb, but it was not impossible to do. They eventually climbed the highest peak in the world by themselves. Whereas if anyone wants to measure the depth of the Mariana trench by themselves, it is practically impossible without the help of any equipment.
Echo sounders are widely used to find the depth of water. It is a mechanism in which we use the echo sounding. Echo sounders are a type of SONAR.
Since you are familiar with the reflection of light and sound, it will be straightforward to understand the working principle of this equipment. It works on the principle of sound. A sound wave with a specific frequency is allowed to transmit into the water. This sound wave will travel through the water and reach the sea bed. Once it touches the sea bed, it will reflect the surface. Ships which have echo sounders have a device called transducers which will transmit and receive this sound wave.

Then the time taken by the wave will be recorded. The velocity of the sound wave is noted parallelly. Thus we can calculate the depth using the equation
\[Depth=velocity\times \dfrac{time}{2}\]………………………(1)
Equation(1) is very familiar to us. Since T is the total time taken by the sound wave to transmit, we need to consider only the time taken to reach the ground. Thus we can take \[{}^{T}/{}_{2}\].Or
\[d=v\times \dfrac{T}{2}\]…………………………(2)
Generally, the velocity of sound in water is approximate \[344{m}/{s}\;\]. For scientific surveys, the temperature, pressure and salinity of the water are also taken into consideration since it affects the wave velocity.
This instrument is mostly used for navigation purposes, for fishing and to map the sea bed.
So option A is the correct answer.

Note: Sonars are used as radars. It will detect the ships and submarines which we have seen in several movies. Echo sounder can detect the presence of fish shoal (a large group ). The frequency range is also different for sonars and echo sounders. And always remember sound travels faster in water than in air.