A. Create a new IAM policy that allows access to those EC2 instances only for the Security team. Apply this policy to the AWS Organizations master account.
B. Create a new tag-based IAM policy that allows access to these EC2 instances only for the Security team. Tag the instances appropriately, and apply this policy in each account.
C. Create an organizational unit under AWS Organizations. Move all the accounts into this organizational unit and use SCP to apply a whitelist policy to allow access to these EC2 instances for the Security team only.
D. Set up SAML federation for all accounts in AWS. Configure SAML so that it checks for the service API call before authenticating the user. Block SAML from authenticating API calls if anyone other than the Security team accesses these instances.