Final answer:
The amplitude of a wave is proportional to its intensity or brightness which is the square of the amplitude. For light waves, this means a higher amplitude indicates a brighter light. However, wave amplitude is independent of wave speed. So, the correct option is C.
Step-by-step explanation:
The relationship between the amplitude of a wave and its intensity or brightness for light is that the amplitude is proportional to the intensity of the wave. More specifically, in the context of light waves, intensity or brightness is associated with the square of the amplitude. A higher amplitude indicates a brighter wave, which means more light energy is being emitted or received. It's important to note that the amplitude of a wave is independent of its speed. The speed of a wave is determined by the medium through which it is traveling and, for electromagnetic waves like light in a vacuum, is a constant (denoted by c) and unaffected by the wave's amplitude.