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.

One laser beam might appear blue and another laser beam might appear red because each laser beam is made up of one specific wavelength of light.

Explanation :

A laser is simply defined as Light Amplification by Stimulated Emission of Radiation.  

Laser light is composed of light of one color only. This means that laser light emits light of single frequency or wavelength. That's why one laser beam appears blue and another laser beam appears red.

So, the correct option is (d) " Each laser beam is made up of one specific wavelength of light ".

a coal burning power plant

the answer would be a