What do derivational morphemes do?

Derivational morphemes are linguistic elements that, when added to a base or root word, alter both the meaning of that word and often its grammatical category. For example, adding the suffix "-ness" to the adjective "happy" transforms it into the noun "happiness," effectively changing the part of speech from an adjective to a noun. Similarly, the prefix "un-" can be added to the adjective "happy" to create "unhappy," changing its meaning to convey the opposite sense.

This process demonstrates how derivational morphemes are instrumental in expanding the lexicon of a language by creating new words while altering their meanings and grammatical functions, which is at the core of their defining characteristic. In contrast, the other options either represent different linguistic concepts or do not accurately encompass the specific role of derivational morphemes.