**Explanation:**

When electromagnetic waves travel in free space, they can undergo various interactions with the surrounding medium or objects. Out of the given options, the correct answer is **attenuation** (option B).

**Attenuation** refers to the reduction in the amplitude or intensity of the electromagnetic wave as it propagates through a medium or space. This reduction can be caused by various factors such as absorption, scattering, or divergence.

Here is a detailed explanation of each option and why they are incorrect:

**a) Absorption:** Absorption occurs when the energy of the electromagnetic wave is absorbed by the medium it is passing through. This can lead to a decrease in the intensity or amplitude of the wave. However, in free space, there is no medium to absorb the electromagnetic waves, so absorption does not occur.

**c) Refraction:** Refraction is the bending of electromagnetic waves as they pass from one medium to another with different refractive indices. This bending occurs due to the change in the speed of the wave in the new medium. However, in free space, there is no change in the medium, so refraction does not occur.

**d) Reflection:** Reflection is the bouncing back of electromagnetic waves when they encounter a surface or boundary between two different media. This bouncing back can occur at various angles depending on the angle of incidence and the properties of the surface. However, in free space, there are no surfaces or boundaries for the waves to reflect off, so reflection does not occur.

Therefore, the only option that is applicable to electromagnetic waves traveling in free space is **attenuation**. As the waves propagate through free space, they can experience a decrease in intensity or amplitude due to factors such as scattering or divergence. However, it is important to note that in a perfect vacuum, where there are no particles or objects to interact with, attenuation is minimal and the electromagnetic waves can travel long distances without significant loss of energy.