The language system is semantics. "Coke" v.s "Sprite." are two different words, so we are focusing on a word. When Anna paused, she was confused because to Melinda, A Coke means a Sprite in her vocabulary. She wanted to confirm the same meaning. Semantics involves a changing meaning depending on the person. She is visiting from Alabama, so the meanings may have chosen just like dialects. Thus, it cannot be phonology/morphology, syntax, or pragmatics.
