Explanation:
To fix this problem you need to stop backup mode for the datafile, so usage of ALTER DATABASE DATAFILE n END BACKUP command is correct answer.
Incorrect Answers:
1: There is incorrect syntax for the ALTER DATABASE command.
2: You don't need to open database with resetting logs after a power failure, it may be required after incomplete recovery.
4: There is incorrect syntax for the ALTER TABLE SPACE command.
Oracle 8, DBA Certification Exam Guide, Jason S. Couchman, p. 588-591
Chapter 12: Overview of Backup and Recovery

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