Answer : 4. "Barometer"
Explanation :

Answer: D) Barometer Explanation: Air Pressure can be defined as the pressure of air in the earth's atmosphere. It reaches a maximum of about 100,000 Pa near sea level and decreases as altitude increases. The barometer is an instrument which is used for measuring Air pressure.   Physics instruments and their uses:   Barkometer  - - - Tanning liquors used in tanning leather Bolometer  - - - Electromagnetic radiation Spectrometer - - -  Properties of light.