How does the speed of electromagnetic energy change with increased altitude in the atmosphere?

The speed of electromagnetic energy in a vacuum, at sea level, or at increased altitudes, remains fundamentally constant, as long as it's in free space. However, the context of electromagnetic wave propagation can cause variations in how we perceive its speed when considering different mediums or atmospheric conditions.

As altitude increases, the density of the atmosphere decreases, leading to less attenuation of electromagnetic waves. This means that while the inherent speed of electromagnetic waves (the speed of light) does not change, the effective speed—considering the medium through which they propagate—might be perceived as increasing due to reduced atmospheric interference.

Nonetheless, the speed does not actually change in the context of electromagnetic theory; rather, it is the clarity and quality of transmission that improves. Therefore, while the idea of a "small uniform increase in speed" may come from the reduction of atmospheric impediments, the core principle is that electromagnetic waves always travel at their designated speed in free space.

This nuanced understanding helps to appreciate why option C is considered the correct choice while keeping in mind that it refers to perceived effectiveness rather than an actual change in the speed of electromagnetic energy.