Final answer:
The common root word that all the words on the left use is 'cred'.
Step-by-step explanation:
The common root word that all the words on the left use is cred.
For example, 'credit' and 'credible' both contain the root word cred which means belief or trust. 'Discredit' and 'credibility' also contain the root word cred but with the addition of prefixes that change the meaning.
Therefore, the correct answer is b) cred.