Explanation:
This statement provides correct syntax to start monitoring the EMPLOYEE_IDX_FK index to determine whether it has been used by an execution plan. Incorrect Answer:
1: This command requires MONITORING USAGE clause with ALTER INDEX command to be used, not MONITOR INDEX clause with ALTER TABLE command.
2: This command requires MONITORING USAGE clause to be used, not MONITORING ON clause.
3: This command requires MONITORING USAGE clause with ALTER INDEX command to be used, not MONITOR ALL INDEXES clause with ALTER TABLE command.
Reference:
OCP Oracle 9i Database: New Features for Administrators, Daniel Benjamin, p. 212-214
Chapter 4: Performance and Scalability Enhancements
Oracle 9i New Features, Robert Freeman, p. 56-59
Chapter 2: Oracle 9i Architecture Changes

---