Final answer:
When auditors vouch a sample of entries in the purchases journal to the supporting documents, they are most likely testing the existence assertion.
Step-by-step explanation:
When auditors vouch a sample of entries in the purchases journal to supporting documents, they are most likely testing the existence assertion. The existence assertion focuses on determining whether the recorded transactions actually occurred and are valid. By verifying the supporting documents, such as purchase orders, invoices, and receipts, auditors can confirm that the purchases were indeed made.