A Company Is Hosting a Web Application on Amazon EC2

A company is hosting a web application on Amazon EC2 instances behind an Application Load Balancer (ALB) . The application has become the target of a DoS attack. Application logging shows that requests are coming from small number of client IP addresses, but the addresses change regularly. The company needs to block the malicious traffic with a solution that requires the least amount of ongoing effort. Which solution meets these requirements?

A) Create an AWS WAF rate-based rule, and attach it to the ALB.
B) Update the security group that is attached to the ALB to block the attacking IP addresses.
C) Update the ALB subnet's network ACL to block the attacking client IP addresses.
D) Create a AWS WAF rate-based rule, and attach it to the security group of the EC2 instances.