| Singular | Plural |
| Auditor | Auditors |
| Auditors' responsibility | Auditor's responsibility |
You don't provide a sentence, so it's difficult to say which article (an, the) should be used. If there is one auditor, put the apostrophe before the s: auditor's responsibility. If there are two or more auditors, put the apostrophe after the s: auditors' responsibility.
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