When will the changes to the settings be effective?

After setting up an EC2 security group with a cluster of 20 EC2 instances, you find an error in the security group settings. You quickly make changes to the security group settings.

When will the changes to the settings be effective?
A . The settings will be effective immediately for all the instances in the security group.
B . The settings will be effective only when all the instances are restarted.
C . The settings will be effective for all the instances only after 30 minutes.
D . The settings will be effective only for the new instances added to the security group.

Answer: A

Explanation:

Amazon Redshift applies changes to a cluster security group immediately. So if you have associated the cluster security group with a cluster, inbound cluster access rules in the updated cluster security group apply immediately.

Reference: http://docs.aws.amazon.com/redshift/latest/mgmt/working-with-security-groups.html