Which AWS service or feature can a company use to apply security rules to a subnet for Amazon EC2 instances?

Explanations:

AWS WAF (Web Application Firewall) is designed to protect web applications by filtering and monitoring HTTP requests, not for applying security rules to subnets.

AWS Shield is a managed DDoS protection service for safeguarding applications, but it does not apply security rules to subnets.

Network ACLs (Access Control Lists) are used to control traffic in and out of a subnet at the network level, allowing for the implementation of security rules for EC2 instances in that subnet.

Security groups act as virtual firewalls for EC2 instances, controlling inbound and outbound traffic at the instance level, but they do not apply rules to a subnet as a whole.