The correct answer is: B. atmospheric pressure.
A barometer is an instrument used to measure atmospheric pressure. It is a simple device that consists of a glass tube filled with mercury and inverted into a dish of mercury. The mercury in the tube will rise or fall depending on the atmospheric pressure.
Option A is incorrect because a barometer is not used to measure pressure in pipes, channels, etc. A manometer is used to measure pressure in pipes, channels, etc.
Option C is incorrect because a barometer is not used to measure very low pressure. A vacuum gauge is used to measure very low pressure.
Option D is incorrect because a barometer is not used to measure the difference of pressure between two points. A differential pressure gauge is used to measure the difference of pressure between two points.
I hope this helps! Let me know if you have any other questions.