What is an affix?

An affix is a linguistic element that is added to a base or root word to modify its meaning or create a new word. Specifically, affixes include prefixes (which are placed at the beginning of a root word) and suffixes (which are added to the end). For example, in the word "unhappiness," "un-" is a prefix that negates the meaning of "happy," and "-ness" is a suffix that turns the adjective "happy" into a noun. Therefore, option B correctly identifies that an affix is a part of a word that changes its meaning.

The other choices do not accurately define an affix. A root word alone refers to the base form of a word without any additions, while a complete sentence is a group of words that expresses a complete thought and does not relate to the concept of affixes. A synonym for a base word refers to a word that has a similar meaning and does not encompass the idea of modifying meanings through affixes.