Explanation:
The data files belonging to a SYSTEM and a rollback segment table spaces can be considered non-essential, because you need a SYSTEM table space to startup database at least and rollback segment table space to rollback not committed information. The loss of SYSTEM table space datafiles or datafiles from table spaces containing rollback segments may cause Oracle to stop running.
Incorrect Answers:
1: Data files belonging to a lost table space cannot be considered non-essential.
2: Data files belonging to an index table space cannot be considered non-essential.
4: Data files belonging to a temporary table space cannot be considered non-essential.
5: Data files belonging to an application data table space cannot be considered non-essential.
Oracle 8, DBA Certification Exam Guide, Jason S. Couchman, p. 676
Chapter 14: Database Failure and Recovery

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