Which of the following has the highest velocity?

1. A

   Cosmic rays

   Cosmic rays are high-energy particles, very fast but still below the speed of light in vacuum. They are not keyed as the highest velocity. Particles cannot outrun c.

2. B

   Light

   (b) Official key: Light has the highest velocity among the listed items. In vacuum, c is about 3 × 10^8 m/s, the relativistic speed limit. Among cosmic rays, light, electrons and supersonic waves, light is the official fastest.

3. C

   Electron

   An electron can approach c in accelerators but does not exceed light in vacuum. Electrons are not the keyed maximum.

4. D

   Supersonic wave

   A supersonic wave is faster than sound in that medium, still orders of magnitude below light. Mach numbers are not the cosmic speed limit.

Summary. Official key is (b). Highest velocity here is light. Cosmic-ray particles and electrons stay below c; supersonic means faster than sound, not faster than light. Keep light. The official letter is (b).