The correct answer is option 'D': speed of light.

Explanation:
Plane electromagnetic waves are a type of wave that consists of oscillating electric and magnetic fields. These waves can travel through a vacuum, which means they do not require a medium to propagate. The velocity of plane electromagnetic waves in a vacuum is equal to the speed of light.

1. Electromagnetic waves:
- Electromagnetic waves are waves that consist of oscillating electric and magnetic fields perpendicular to each other and to the direction of wave propagation.
- These waves are produced by the acceleration of charged particles.
- Examples of electromagnetic waves include radio waves, microwaves, infrared waves, visible light, ultraviolet waves, X-rays, and gamma rays.

2. Characteristics of electromagnetic waves:
- Electromagnetic waves do not require a medium to propagate. They can travel through empty space, such as a vacuum.
- The speed of electromagnetic waves in a vacuum is constant and is denoted by the symbol 'c'.
- The speed of light in a vacuum is approximately 3 x 10^8 meters per second (m/s).

3. Relationship between electromagnetic waves and light:
- Light is a form of electromagnetic radiation that is visible to the human eye.
- The speed of light in a vacuum is equal to the speed of electromagnetic waves in a vacuum.
- Therefore, the velocity of plane electromagnetic waves in a vacuum is equal to the speed of light.

Conclusion:
In conclusion, the velocity of plane electromagnetic waves in a vacuum is equal to the speed of light. This is because electromagnetic waves can propagate through a vacuum and the speed of light in a vacuum is constant. It is important to note that this velocity is much greater than the speed of sound or any other subsonic or supersonic velocities.