The correct answer is: B. Optical attenuator.
An optical attenuator is a device that reduces the intensity of light. It is used in optical fiber communications to control the power of the light signal. Optical attenuators can be made of a variety of materials, including glass, plastic, and metal. They are typically used in conjunction with optical amplifiers to ensure that the power of the light signal remains within a safe and acceptable range.
Option A, Compressor, is incorrect because a compressor is a device that reduces the volume of a gas or liquid.
Option C, Barometer, is incorrect because a barometer is a device that measures atmospheric pressure.
Option D, Reducer, is incorrect because a reducer is a device that decreases the size of something.