What characteristic do vowel sounds not change under in Spanish?

In the context of vowel sounds in Spanish, phonology refers to the way sounds function within a particular language. Vowel sounds in Spanish are consistent and do not change based on the phonological rules of the language. For instance, the five basic vowel sounds in Spanish—/a/, /e/, /i/, /o/, and /u/—are stable and remain the same regardless of their position within a word or surrounding consonants.

This consistency in vowel sounds is a significant feature of Spanish phonology that distinguishes it from languages where vowel sounds may morph or alter depending on various factors. Therefore, phonology is the correct answer as it encompasses the predictable, unchanging nature of Spanish vowel sounds across different contexts and usages, highlighting their reliable pronunciation and function in the language.