Explanation:
When a statement is parsed and the resulting execution plan contains a reference to the index an index will be considered "used".
Incorrect Answers:
1: It will be considered as “used” when a statement is parsed and the resulting execution plan contains a reference to the index, not only when the index is specified in a hint.
2: An index will be considered "used" when a statement is parsed and the resulting execution plan contains a reference to the index, not when data is fetched by an execution plan that uses that index.
3: It will be considered as “used” when a statement is parsed and the resulting execution plan contains a reference to the index, not when a statement is executed and the execution plan contains a reference to the index.
Reference:
OCP Oracle9i 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: Oracle9i Architecture Changes

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