What is defined as the amount of energy in a light or sound wave, perceived as brightness or loudness?

The correct choice relates to the concept of intensity, which is the amount of energy carried by a light or sound wave. In optics and acoustics, intensity is directly associated with how we perceive the brightness of light or the loudness of sound. For instance, brighter lights have higher intensity, and louder sounds correspond to greater intensity levels.

Amplitude, while also related, refers specifically to the maximum extent of a wave's oscillation and is a critical factor in determining intensity. However, intensity captures the overall perception of brightness or loudness more comprehensively, as it encompasses the energy conveyed by those amplitude variations.

Frequency, on the other hand, relates to the pitch of a sound or the color of light, rather than its energy level. Color pertains to visual perception, specifically which wavelengths of light are reflected or emitted, but not to how bright the light appears. Thus, intensity stands out as the most fitting definition in relation to perceived energy levels in waves.