The correct answer is: a word part added before or after a root word.
An affix is a word part that is added before or after a root word to create a new word with a modified meaning. It can be a prefix, which is added before the root word, or a suffix, which is added after the root word. An affix does not have a meaning on its own, but it modifies the meaning of the root word it is attached to. So, option D - "a word part added before or after a root word" - is the correct definition of an affix.
