Answer:
Prefix
Explanation:
The root is the base word.
An Affix added on the the root word helps change it's meaning.
A prefix is a type of affix that is placed in front of a word to change it's meaning (example: un-, which means not, added on to the word "happy" makes "unhappy").
A suffix is a type of affix added on to the end of the root word (example: -er, which means one who preforms the action, added on to "read" creates "reader"
