Which AWS CloudFormation feature should be used to define a collection of resources grouped as a single unit?

The correct choice is to use StackSet when defining a collection of resources grouped as a single unit in AWS CloudFormation. StackSets provide a way to manage a collection of CloudFormation stacks that can be deployed across multiple accounts and regions. This is particularly useful for deploying the same resources uniformly in different environments while ensuring that they are managed cohesively.

StackSets enable administrators to create a template and then deploy it across a set of accounts and regions with a single operation. This not only streamlines the deployment process but also enhances consistency, as it ensures all stacks are created based on the same configuration and parameters.

Examine other features: Exports are utilized for sharing output values between stacks, but they do not encapsulate resources as a unit. A Policy in AWS CloudFormation can define specific permissions for resources, but again, it does not group resources together as a coherent unit for deployment. Change sets are useful for previewing changes to be made to a stack before applying them, but they do not define collections of resources.

Therefore, StackSet is the feature that best fits the requirement of encapsulating a collection of resources as a single unit in CloudFormation.