Four instruments have been given, out of which three are alike in some manner and one is different. Select the one that is different from the rest:

Deciblemeter

The correct answer is Option (2) → Deciblemeter

To identify the odd one out, we must look at what each instrument measures:

• Udometer: An instrument used to measure the amount of liquid precipitation (rain) over a set period of time.
• Ombrometer: Another name for a rain gauge, used to measure rainfall.
• Pluviometer: Also a technical term for a rain gauge.
• Deciblemeter (Decibel meter): An instrument used to measure sound intensity or noise levels in decibels ($dB$).

Conclusion

Udometer, Ombrometer, and Pluviometer are all synonyms for the same device—a rain gauge. The Deciblemeter is the only instrument in the list used for measuring sound, making it the different one.