Which of the following types of fraud tests would be most effective if an internal...

The most effective type of fraud test for looking for possible fictitious vendors would be running checks to uncover post office box addresses that match employee addresses. This test directly targets a common tactic used in vendor fraud schemes, where employees might set up fictitious vendor accounts and direct payments to themselves via P.O. boxes.

IIA resources on auditing for fraud, which often discuss various methods for detecting fictitious vendors, including address comparisons.