Explanation:
Actually there are some answers possibly correct. But based on question it looks like it was incomplete recovery performed when the log sequence number should be reset. To reset the log sequence number when opening a database after recovery and thereby create a new incarnation of the database, execute the following statement:

 ALTER DATABASE OPEN RESET LOGS; 

Incorrect Answer:
1: This command can be used after recovery also, but only after media recovery.
2: This command can be used for clearing an unachieved log allows it to be reused without archiving it. This action makes backups unusable if they were started before the last change in the log, unless the file was taken offline prior to the first change in the log. Hence, if you need the cleared log file for recovery of a backup, then you cannot recover that backup. Also, it prevents complete recovery from backups due to the missing log
4: To preserve the log sequence number when opening a database after media recovery, execute either of the following statements:

 ALTER DATABASE OPEN NORESET LOGS; 
 ALTER DATABASE OPEN; 

Reference:
OCP Oracle 9i Database: Fundamentals II Exam Guide, Rama Velpuri, p. 336-337
Chapter 14: User-Managed Incomplete Recovery

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