NACL acts as a firewall at the subnet level of the VPC and we can deny the offending IP address block at the subnet level using NACL rules to block the incoming traffic to the VPC instances. Since NACL rules are applied as per the Rule numbers make sure that this rule number should take precedence over other rule numbers if there are any such rules that will allow traffic from these IP ranges. The lowest rule number has more precedence over a rule that has a higher number.
The IAM Documentation mentions the following as a best practices for IAM users
For extra security, enable multi-factor authentication (MFA) for privileged IAM users (users who are allowed access to sensitive resources or APIs). With MFA, users have a device that generates a unique authentication code (a one-time password, or OTP). Users must provide both their normal credentials (like their user name and password) and the OTP. The MFA device can either be a special piece of hardware, or it can be a virtual device (for example, it can run in an app on a smartphone).
Options C is invalid because these options are not available
Option D is invalid because there is not root access for users
For more information on IAM best practices, please visit the below URL:
https://docs.IAM.amazon.com/IAM/latest/UserGuide/best-practices.html
The correct answer is: Modify the Network ACLs associated with all public subnets in the VPC to deny access from the IP Address block.
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