What are added to words to create related but different words, such as -ness in happiness?

The correct choice refers to elements that are attached to the base form of words to create new words with related meanings. In this context, the term indicates that these modifications to the root or base word serve to expand its usage and impart additional meaning.

Affixes include both prefixes and suffixes. The example given, such as "-ness" in "happiness," showcases how a suffix can transform an adjective (happy) into a noun (happiness), indicating a state or quality. This construction highlights the role of affixes in word formation. Therefore, labeling "-ness" as an affix is accurate, as it is specifically a type of affix known as a suffix that modifies the meaning of the root word.

Understanding affixes is essential for grasping how the English language evolves and how nuanced meanings can be conveyed through the addition of these linguistic elements. Thus, the choice reflects the broad category that encompasses both prefixes and suffixes, making it the most encompassing and correct option.