Most children, even before starting school education, have already internalized the _____ and _____ of language and possess full linguistic competence.

Before starting school, children have already internalized the complexities (जटिलताओं) and rules (नियमों) of their language naturally through immersion. This reflects the position of NCF 2005 and Chomsky's nativism—children master grammar implicitly before formal schooling. So the correct pair is जटिलताओं, नियमों.