What is the term for energy in the spectrum of a vowel sound?

The correct term for energy in the spectrum of a vowel sound is "formant." Formants are specific frequency ranges in the sound spectrum that are associated with different vowel sounds. They occur due to the resonant frequencies of the vocal tract when producing speech sounds. Each vowel sound has a distinct pattern of formants that allow listeners to differentiate between them.

In terms of other options, amplitude refers to the loudness of a sound, pitch relates to the perceived frequency of a sound, and timbre describes the quality or color of a sound that makes it possible to distinguish between different sound sources, even if they have the same pitch and loudness. While all these terms are related to sound, formants specifically capture the unique energy characteristics of vowel sounds in the acoustic spectrum.