For the same frequency, wavelength λ = v/f; since light speed is vastly greater than sound speed, the EM wavelength is much larger than the ultrasonic wavelength.

All bodies with temperature above absolute zero emit electromagnetic radiation (thermal radiation).

v/c = Δλ/λ = 4/6000 ≈ 6.667×10^-4 so v ≈ 6.667×10^-4 × 3×10^8 m/s ≈ 2×10^5 m/s.

LEDs (light-emitting diodes) emit visible light as they are p–n junction devices designed for that spectral region.

X-rays are a form of electromagnetic radiation, so 'electromagnetic waves' is correct.

The Brackett and Pfund series correspond to transitions that emit in the infrared portion of the hydrogen spectrum.

All components of electromagnetic radiation travel at the same speed c in vacuum, so option D is correct.

Increasing the accelerating voltage raises photon energies, producing more penetrating X-rays.

Laser wavelengths provide very stable and precise optical standards that can be used to define or measure length with high accuracy.

X-rays are produced when high-speed electrons strike the anode (anti-cathode) and decelerate, producing bremsstrahlung and characteristic X-rays.