What is the main function of a security group in AWS?

The primary function of a security group in AWS is to act as a virtual firewall for EC2 instances. Security groups define the inbound and outbound traffic rules for instances, effectively controlling what traffic is allowed to reach the instance and what traffic can leave it. By specifying allowed protocols, port ranges, and source/destination IP address ranges, security groups provide a way to enforce security policies at the instance level. This mechanism is crucial for protecting resources in a cloud environment, enabling administrators to isolate instances and only allow necessary traffic.

In this context, other options do not align with the core functionality of security groups. Controlling bandwidth for network traffic pertains more to network optimization rather than access control. Monitoring application performance relates to application management and performance tuning, while providing network redundancy is about ensuring high availability and fault tolerance, neither of which are functions carried out specifically by security groups.