Explanation:

Electromagnetic waves are classified according to their frequency or wavelength. The different types of electromagnetic waves are radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays. The energy of electromagnetic waves increases as the frequency increases.

The energy of electromagnetic waves can be calculated using the formula:

E = hf

where E is the energy, h is Planck's constant, and f is the frequency.

For the given question, the energy of the electromagnetic waves is 15 keV. The energy can be converted to frequency using the above formula.

E = 15 keV = 15 x 10^3 eV = 1.5 x 10^4 eV

h = 6.626 x 10^-34 J s = 4.136 x 10^-15 eV s

f = E/h = (1.5 x 10^4 eV)/(4.136 x 10^-15 eV s) = 3.63 x 10^18 Hz

The frequency of the electromagnetic waves is 3.63 x 10^18 Hz. This frequency corresponds to X-rays.

Therefore, the correct answer is option 'C', X-rays.

Summary:

- Electromagnetic waves are classified according to their frequency or wavelength.
- The energy of electromagnetic waves increases as the frequency increases.
- The energy of electromagnetic waves can be calculated using the formula E = hf.
- The energy of the given electromagnetic waves is 15 keV.
- The frequency of the electromagnetic waves is 3.63 x 10^18 Hz.
- This frequency corresponds to X-rays.
- Therefore, the correct answer is option 'C', X-rays.