Speed of Electromagnetic wave is not the same:

The correct answer is Option (1) → in all media

In vacuum every electromagnetic wave travels with the same speed \( c = 1/\sqrt{\mu_0\varepsilon_0} = 3\times10^{8}\,\mathrm{m/s} \), whatever its frequency, wavelength or intensity. So the speed is the same for all intensities, frequencies and wavelengths in vacuum (Options 2, 3 and 4 are true statements).

In a material medium the speed is \( v = c/n \), where the refractive index n depends on the medium (and on the frequency), so the speed of an electromagnetic wave is not the same in all media. Hence the statement “not the same in all media” is the correct completion.

NCERT (2026-27):
Nature of Electromagnetic Waves – “… These replace \( \varepsilon_0 \) and \( \mu_0 \) in the description to electric and magnetic fields in Maxwell’s equations with the result that in a material medium of permittivity \( \varepsilon \) and magnetic permeability \( \mu \), the velocity of light becomes,

    \( \displaystyle v = \frac{1}{\sqrt{\mu\varepsilon}} \qquad (8.11) \)

Thus, the velocity of light depends on electric and magnetic properties of the medium. … It has been shown by experiments on electromagnetic waves of different wavelengths that this velocity is the same (independent of wavelength) to within a few metres per second, out of a value of 3×10⁸ m/s.”