Question

Which best explains why one laser beam might appear blue and another laser beam might appear red?

The energy in laser beams is concentrated.
The energy in laser beams is scattered.
Each laser beam is made up of all visible wavelengths of light.
Each laser beam is made up of one specific wavelength of light.

Answer 1

Each laser beam is made up of one specific wavelength of light.

Lasers are (approximately) monochromatic: their light is concentrated at a single wavelength (or a very narrow band). Different wavelengths correspond to different perceived colors (shorter ≈ blue, longer ≈ red), so a blue laser emits a shorter wavelength than a red laser.