Answer:
The common root word that they all have in common is "cred".
Explanation:
The root word "cred" is present in several words used in the English language. Some examples are: incredible, credulous, credential, credible, credence, and credit. "Cred" comes from Latin, and its original meaning is "believe". Notice how all the words that have "cred" as their root are somehow related to the sense of "believe". If something is incredible, that means it is hard to believe it is true. If you show your credentials, you are proving your identity, so that someone else will believe you are who you are.
