Speed of Electromagnetic wave is Not the same:

The correct option is in all media.

Let's break down the physical principles step-by-step to understand why the speed of an electromagnetic wave is not the same in all media:
1. Speed in a Vacuum: In a vacuum (free space), all electromagnetic waves, regardless of their wavelength, frequency, or intensity, travel at the same constant speed. This speed is a fundamental physical constant denoted by c, which is approximately $c\approx 3\times {10}^8{\text{ms}}^{-1}$.
Therefore, the speed remains identical in a vacuum for all frequencies, wavelengths, and intensities.

2. Speed in Material Media: When an electromagnetic wave enters a material medium (such as water, glass, or air), it interacts with the atoms and molecules of that medium. This interaction slows down the propagation of the wave.
The speed of light in a medium (v) is given by the relation:
$v=\frac{c}{n}$
where n is the refractive index of the medium.

3. Dependence on the Medium: Since different media have different optical densities and electrical/magnetic properties (permitivity and permeability), their refractive indices (n) are different. For example, the refractive index of water is approximately 1.33, while that of glass is about 1.5. As a result, the speed of electromagnetic waves changes when transitioning from one medium to another, making it not the same in all media.