Acoustic Rain Gauge: Introduction
An acoustic rain gauge is a device used to measure rainfall by using sound waves. It is an innovative and non-contact method for measuring precipitation. The gauge is primarily based on the principle that raindrops falling into water create sound waves that can be detected and analyzed to determine the amount of rainfall.

Working Principle of Acoustic Rain Gauge
The acoustic rain gauge consists of a hydrophone, which is a special type of microphone designed to pick up underwater sound signals. The hydrophone is usually placed at the bottom of a water-filled container, such as a bucket or a tank. When raindrops fall into the water, they generate acoustic signals that are sensed by the hydrophone.

Hydrophone as an Alternative Name for Acoustic Rain Gauge
The correct answer, option 'C', states that an acoustic rain gauge can also be known as a "hydrophone." This is because the hydrophone is the main component of the acoustic rain gauge. The hydrophone detects the sound waves produced by the raindrops falling into the water, making it an integral part of the gauge.

Other Options
The other options provided in the question are not correct alternatives for an acoustic rain gauge. Here is a brief explanation of why they are not suitable:

a) Hydro gauge: This term is not commonly used to refer to an acoustic rain gauge. The term "hydro gauge" is more commonly associated with pressure gauges used in hydraulic systems.

b) Hydro remote: This term does not accurately describe an acoustic rain gauge. It may refer to remote sensing techniques used in hydrology, but it is not a specific term for an acoustic rain gauge.

d) Hydro cable: This term is unrelated to an acoustic rain gauge. "Hydro cable" may refer to cables used in hydroelectric power systems or underwater communication systems, but it does not pertain to the measurement of rainfall.

Conclusion
In conclusion, an acoustic rain gauge is a device that utilizes a hydrophone to measure rainfall based on the sound waves produced by raindrops falling into water. The hydrophone is the main component of the gauge, making "hydrophone" an appropriate alternative name for an acoustic rain gauge.