Analysis of each instrument:
Udometer: An instrument used to measure rainfall. The term derives from Latin "udus" meaning wet or moist.
Deciblemeter: An instrument used to measure sound intensity or loudness, expressed in decibels.
Ombrometer: An instrument used to measure rainfall. The term derives from Greek "ombros" meaning rain.
Pluviometer: An instrument used to measure rainfall. The term derives from Latin "pluvius" meaning rain.
Three instruments (Udometer, Ombrometer, and Pluviometer) measure the same physical quantity: rainfall.
One instrument (Deciblemeter) measures a completely different physical quantity: sound intensity.
Therefore, the Deciblemeter is different from the rest.
The correct answer is Option 2: Deciblemeter.