The liquid employed in aneroid barometer.

An aneroid barometer is a type of barometer that doesn't use liquid to measure atmospheric pressure.
It uses a small, flexible metal box called an aneroid cell.
This box is partially evacuated (air removed), so changes in atmospheric pressure cause it to expand or contract.
These movements are mechanically amplified and displayed on a dial to indicate the pressure.

Since it relies on a mechanical system and not a liquid column, the answer is "None".
Options A (Mercury), B (Water), and C (Petrol) are all liquids but are not used in aneroid barometers.
Mercury is used in mercury barometers, which is a different type of barometer.