Which stage of development includes mastering the use of complex sentences?

The stage of development that includes mastering the use of complex sentences is the syntactic stage. In this stage, children typically demonstrate greater proficiency in understanding and producing complex grammatical structures. This often enables them to link ideas more coherently and express more nuanced thoughts through intricate sentence formations.

The phonemic stage focuses on the awareness and manipulation of sounds within words, primarily dealing with phonological awareness. While important for literacy, it doesn't directly address sentence complexity.

The morphophonemic stage involves understanding and using morphemes, which are the smallest units of meaning in a language. While it may advance a child's grammatical proficiency, it does not specifically emphasize the construction of complex sentences.

The pragmatic stage centers on the social aspects of language use, such as conversation and context-appropriate language. Although this stage increases awareness of how language is used in various situations, it does not primarily concern the formation of complex sentences.

Thus, focusing on syntax is crucial for developing the ability to construct complex, multi-clause sentences, making it the clear answer in this context.