The correct answer is: D. difference of pressure between two points.
A manometer is a device used to measure pressure differentials. It is typically used to measure the pressure of a fluid in a pipe or channel, but it can also be used to measure the pressure of a gas or vapor. The manometer consists of a U-shaped tube filled with a liquid, such as mercury or water. One end of the tube is connected to the system whose pressure is to be measured, and the other end is open to the atmosphere. The difference in the height of the liquid in the two legs of the tube is proportional to the pressure difference between the system and the atmosphere.
Option A is incorrect because a manometer can be used to measure the pressure of a fluid in a pipe or channel, but it can also be used to measure the pressure of a gas or vapor.
Option B is incorrect because a manometer is used to measure the difference of pressure between two points, not the atmospheric pressure.
Option C is incorrect because a manometer can be used to measure very low pressures, but it is not specifically designed for that purpose.