Electromagnetic waves can be best described as:

The best description of electromagnetic waves is that they are produced by oscillating electric charges. This means that when electric charges accelerate, they create changing electric and magnetic fields that propagate through space, forming electromagnetic waves.

Electromagnetic waves include a range of types such as radio waves, microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays. They do not require a medium, such as air or water, to travel; instead, they can move through the vacuum of space. This characteristic distinguishes them from mechanical waves, which do need a medium to propagate.

While it is true that electromagnetic waves have repeating patterns over a wavelength, this characteristic does not encompass the full nature of how these waves are formed and transmitted. Understanding the role of oscillating electric charges is crucial in grasping the fundamental principle behind electromagnetic wave phenomena, making it the most accurate descriptor among the options provided.