What phenomenon occurs in a vacuum?

In a vacuum, the defining characteristic is the absence of air or matter. A vacuum is a space devoid of particles, which leads to the absence of atmosphere and sound conduction. This distinction is crucial because many physical phenomena rely on the presence of matter. For example, sound requires a medium (like air, water, or solids) to propagate; without any medium, sound cannot travel at all.

The other options do not accurately describe the characteristics of a vacuum. Sound does not travel perfectly in a vacuum; in fact, it cannot travel at all due to the lack of a medium. Light speed is constant in a vacuum, specifically at approximately 299,792 kilometers per second, rather than being reduced. Matter being compressed does not occur in empty space as there is simply no matter to compress. Thus, stating that no air or matter is present effectively captures the essence of what a vacuum is.