If you asked your friend to hand you a book and he handed you a pencil instead, which language system most likely broke down?
phonology
morphology
syntax
semantics
pragmatics

If you asked your friend to hand you a book and he handed you a pencil instead, it is phonology as a language system that is most likely broken down. Phonology deals with systems of sounds which includes phonetics in a specific language.

The answer would be the first choice.