Final answer:
An accelerometer measures the rate at which velocity changes, which is the definition of acceleration, and it is found in many modern devices for various sensing applications.
Step-by-step explanation:
The best definition of what an accelerometer does is C, it measures velocity in a given direction. More specifically, an accelerometer is a device that measures the rate at which velocity changes, which is the definition of acceleration. Accelerometers are commonly found in many modern devices and are used for a variety of applications, such as detecting the orientation of a smartphone, measuring the intensity of vibrations, or monitoring dynamic movements in a vehicle. It should not be confused with devices that calculate the rotation of a device, which are gyroscopes, or notions like prolonging battery life or making the device run faster, which are unrelated to what an accelerometer does.