The correct answer is A, Network ACLs. Network ACLs (Access Control Lists) operate at the subnet level and provide a way to control inbound and outbound traffic for resources within that subnet. They are stateless, meaning they do not allow for a response traffic to automatically be allowed; rules must be explicitly defined for both incoming and outgoing traffic. Unlike security groups, which are attached to individual instances and can have stateful rules, Network ACLs are a more granular control mechanism at the subnet level. IAM is used for managing access to AWS resources and does not control network traffic. AWS WAF is specifically for filtering web traffic and is not the correct answer in this context as the question is about general traffic control within a VPC.