The question asks to identify the range to which light with a wavelength of 15 nm belongs. To answer this, we need to understand the different regions of the electromagnetic spectrum.

Option 1: X-ray

X-rays have very short wavelengths, ranging from 0.01 nm to 10 nm. The given wavelength of 15 nm does not fall within the X-ray range.

Option 2: Gamma ray

Gamma rays have even shorter wavelengths, typically less than 0.01 nm. Again, the given wavelength of 15 nm does not fall within the gamma ray range.

Option 3: Infrared

Infrared light has longer wavelengths compared to visible light, ranging from about 700 nm to 1 mm. The given wavelength of 15 nm is much shorter than the infrared range.

Option 4: Ultraviolet

Ultraviolet light has shorter wavelengths than visible light, ranging from about 10 nm to 400 nm. The given wavelength of 15 nm falls within the ultraviolet range.

Therefore, the correct answer is option 4: Ultraviolet.