What does the term "morphological" refer to in language?

The term "morphological" pertains specifically to the structure and formation of words within a language. Morphology is the study of the internal structure of words and how they can be modified or built from morphemes, which are the smallest meaningful units of language.

In this context, "the formation of permissible words in a language" accurately captures the essence of morphology, as it involves understanding how different prefixes, suffixes, and root words can be combined to create valid words in that language. This includes rules for word formation, variations of words to indicate tense, number, aspect, and other grammatical nuances.

The other choices address different aspects of language. Sentence structure is related to syntax, which deals with how sentences are constructed. The meaning of vocabulary words pertains to semantics, the study of meaning in language. Pronunciation involves phonetics and phonology, which focus on the sounds in speech rather than the formation or structure of words. Therefore, the correct answer highlights the specific linguistic focus of morphology.