C

Step 1: Understanding Decibel: The decibel (dB) is a unit that expresses the ratio of two values of a physical quantity, often power or intensity. It is commonly used to measure sound levels.
Step 2: Decibels and Sound: In the context of sound, the decibel scale measures sound loudness, which relates to the intensity of sound perceived by the human ear. An increase of 10 dB represents a tenfold increase in intensity, which roughly translates to perceived loudness doubling.
Step 3: Analysis of Options:
- Option A: Intensity of light: This is not measured in decibels; light intensity is typically measured in lumens or watts.
- Option B: X-rays radiation capacity: X-ray intensity may involve different units, and decibels are not standard for this measurement.
- Option C: Sound loudness: This option correctly aligns with the primary use of decibels. It quantifies how loud sounds are to humans.
- Option D: Energy of radiation: This is typically measured in joules or electronvolts, not in decibels.
Therefore, the correct answer is C.