The correct answer is: B. Ocean depth
A fathometer is an instrument used to measure the depth of water. It is also known as a depth sounder or echo sounder. Fathometers work by emitting a sound wave and then measuring the time it takes for the sound wave to travel to the bottom of the water and back. The depth of the water can then be calculated based on the speed of sound in water and the time it takes for the sound wave to travel.
Rainfall is measured using a rain gauge. A rain gauge is a device that measures the amount of rain that falls in a given area. Rain gauges typically consist of a funnel that collects the rain and a container that measures the amount of water that is collected.
Sound intensity is measured using a sound level meter. A sound level meter is a device that measures the intensity of sound. Sound level meters typically consist of a microphone that converts sound waves into electrical signals and a display that shows the intensity of the sound.
Earthquakes are measured using a seismograph. A seismograph is a device that measures the ground motion caused by an earthquake. Seismographs typically consist of a sensor that detects the ground motion and a recording device that records the ground motion.