Answer:
Language acquisition device
Step-by-step explanation:
According to Noam Chomsky, every infant has the innate ability to gain and produce language. He also suggests that an infant would not be able to learn a language as fast as they do unless they had innate knowledge of grammar.
Here, Susan has started to speak in sentences. This means that she has understood how grammar is to be used correctly.
Hence, according to Noam Chomsky she has gained this ability due to her innate grammatical knowledge which he describes as Language acquisition device.