Answer:
sub-
Explanation:
Remember that a prefix is a group of letters placed before the root of a word in order to change it's meaning.
In this case, the prefix sub- means something bellow or under something.
For example: the root word way means a road or a track, by adding the prefix sub- a new word is formed, subway meaning an underground railway.
Here by using this prefix with the word text, subtext is formed, refering to an underlying meaning.
