Explanation:
This command can be used to alter the active size of the SGA dynamically.
Incorrect Answers:
1: This command is incorrect, because the SGA_MAX_SIZE cannot be changed dynamically. And question was about SGA size dynamic change, not SGA_MAX_SIZE.
2: This command is incorrect. You cannot drop the DB_CACHE_SIZE parameter.
3: This command is incorrect. You cannot set the DB_CACHE_SIZE parameter as AUTO.
Reference:
OCP Oracle9i Database: New Features for Administrators, Daniel Benjamin, p. 180-182
Chapter 3: Manageability Enhancements
Oracle 9i New Features, Robert Freeman, p. 16-17
Chapter 1: Oracle9i Database Administration and Management Features

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