What is measured by manometer?

Answer: Option B

The correct answer is B: Pressure of gas.
Let's break down why:

Think of a manometer like a simple tool for comparing pressures.
It usually involves a U-shaped tube filled with a liquid.

Air pressure (Option A): While manometers can *relate* to air pressure, they are specifically designed to measure the pressure of enclosed gases.

Pressure of gas (Option B): A manometer directly measures the pressure of a gas within a container or system.
It works by comparing the gas pressure to a reference pressure (often atmospheric pressure).
The difference in liquid levels in the U-tube tells us the pressure difference.

Density of liquids (Option C): Density is a measure of how much mass is in a given volume.
Manometers don't directly measure density; instruments like hydrometers do that.

Pressure of oil on the surface (Option D): While pressure exists on the surface of a liquid like oil, a manometer isn't typically used for this specific measurement.
Other sensors are more suitable for measuring surface pressures.

In summary, a manometer is primarily used to measure the pressure of a gas by comparing it to a known reference pressure.