Which one of the following is not correctly matched?

Answer: Option C

To determine which option is not correctly matched, let's analyze each option:

Option A: Decibel ⟺ Unit of sound intensity
This is correct. The decibel (dB) is indeed the unit used to measure the intensity of sound.

Option B: Horsepower ⟺ Unit of power
This is also correct. Horsepower (hp) is a unit of power commonly used to measure the output of engines and motors.

Option C: Nautical miles ⟺ Unit of naval
This is incorrect. Nautical miles (nmi) are a unit of distance, not a unit specifically related to naval operations. Naval operations may use nautical miles for distance measurements, but nautical miles themselves are not a unit of naval.

Option D: Celsius ⟺ Unit of heat
This is incorrect. Celsius (°C) is a unit of temperature, not a unit of heat. Heat is measured in joules or calories.

Therefore, the option that is not correctly matched is Option C.