Answer: D. Affix
Let us define a few key terms:
Root - A root word is a word that essentially, stands by itself. An example of this could be: "read". The word itself indicates no particular action and is called a root because it can be the basis or stem of others: such as "reading".
Prefix - As the "pre" in prefix suggests, this addition comes before or in this context, "the beginning" of the word itself. An example would be possible. By adding the prefix "Im" in front of possible it now becomes an entirely new word, "Impossible."
Suffix - We can treat this as essentially the opposite of a prefix, as the alteration occurs after the word itself. For instance, the word "end" would have the suffix "less" to create endless. In a way, giving the word a new meaning.
Affix - This is essentially the composite whole of both Prefixes and Suffixes.
Since the question says A ___ is added to the beginning or end of a word to change its meaning.
It is essentially asking for a word that means both a prefix and a suffix - Which is technically given to us through the word "affix"
We can come to the conclusion that our answer is D. Affix