You want to perform some maintenance on your ASM disk groups. As a result, you want to prevent RDBMS instances from connecting to the ASM instance, but you do not want to shut down the ASM instance. What command will achieve this objective?

Answer: A

Explanation: Running ALTER SYSTEM ENABLE RESTRICTED SESSION prevents connections from RDBMS instances. B is incorrect because SUSPEND is valid only for an RDBMS instance. C is incorrect because KILL SESSION will disconnect a user's session, but it will not prevent new connections. D is incorrect because this command is not syntactically correct.