Vowel reduction is frequently mischaracterised as phonetic sloppiness, yet phonologically it is a systematic rule of connected speech. What follows from this distinction for the listener's perception of spoken English?

D. It provides contrast that marks the language's stress pattern

Reduction is not an acoustic defect requiring contextual repair, nor is it confined to function words; unstressed syllables in content words reduce systematically. Crucially, acoustic timing in natural speech is gradient rather than strictly metronomic, making vowel quality contrasts vital for metrical parsing.