What is a Security Group?

A.
None of these.

B.
A list of users that can access Amazon EC2 instances.

C.
An Access Control List (ACL) for AWS resources.

D.
A firewall for inbound traffic, built-in around every Amazon EC2 instance.

Explanation:
A security group acts as a virtual firewall for your instance to control inbound and outbound traffic. http://
docs.aws.amazon.com/AmazonVPC/latest/UserGuide/VPC_SecurityGroups.html