Which AWS service or feature acts as a firewall for Amazon EC2 (CLF-C02)

Correct answer: D) Security group.
Security groups act as a virtual firewall for your Amazon EC2 instances to control inbound and outbound traffic. They are stateful, meaning if you send a request from your instance, the response traffic for that request is allowed to flow in regardless of inbound security group rules. Security groups are associated with EC2 instances and are defined by a set of rules that specify the allowed inbound and outbound traffic.

Here's a brief overview of how security groups work:

Inbound Rules: Define the incoming traffic that is allowed to reach the instances. You can specify the protocol (TCP, UDP, ICMP), port range, and source (which can be another security group, a network ACL, or a CIDR IP range).
Outbound Rules: Define the outgoing traffic from the instances. By default, all outbound traffic is allowed. You can restrict outbound traffic by specifying the protocol, port range, and destination.
Security groups are an essential part of securing your instances in the AWS Cloud, providing a layer of protection against unauthorized access.