The correct answer is in-.
Prefixes are parts of words that are added BEFORE the root word (the main part of a word), unlike suffixes, which are added AFTER. So, if we take a look at the adjective incredible, we can see a couple of things.
First of all, the root word is cred (comes from Latin verb credere which means to believe). The part before it is the prefix in-; the part after it is the suffix -ible. The prefix makes this word a negative one; the suffix makes it an adjective.
