What defines how to launch Amazon EC2 instances in Amazon Auto Scaling?

The determination of how to launch Amazon EC2 instances within Auto Scaling groups is primarily handled by a Launch Configuration. A Launch Configuration is a template that specifies the instance type, Amazon Machine Image (AMI), key pair, security groups, and other parameters required for launching EC2 instances within the Auto Scaling group. This template allows the Auto Scaling service to create and configure instances consistently based on the defined settings when scaling out to meet demand or scaling in to reduce capacity.

In contrast, the other options serve different purposes in the AWS ecosystem. An Instance Profile is used to associate permissions with the instances, allowing them to access AWS resources securely but does not define the launch configurations for new instances. A Security Group acts as a virtual firewall to control inbound and outbound traffic for your instances, but it is not responsible for determining the instance launch parameters. An Auto Scaling Policy defines the conditions under which scaling actions occur, such as when to add or remove instances, but it does not dictate how those instances should be launched.