Words given on the left side of (::) are related  with each other by some Logic/Rule/relation. Select the missing word/word pair on the right side of (::) from the given alternatives based on the same Logic/Rule/Relation.

Odometer : Distance Travelled by a Vehicle :: Barometer : ?

Atmospheric pressure

Logic here is :

Odometer : Distance Traveled by a Vehicle

Odometer is used to measure Distance traveled by a vehicle .

Similarly ,

Barometer : ?

Barometer is used to measure Atmospheric Pressure .

Atmospheric Pressure is correct answer .