The correct choice is !GetAtt because it retrieves an attribute from a resource such as an Amazon RDS DB instance's Endpoint which you can place in the Outputs for other stacks to consume.

!GetAtt returns named attributes that CloudFormation populates after the resource is created and Endpoint is one of those attributes for RDS instances and for Aurora clusters. You can expose that endpoint in the Outputs section and then export or import it so other stacks can consume the connection endpoint.

!Sub is for string substitution and interpolation and it cannot by itself fetch runtime attributes from resources. You can combine !Sub with other intrinsics to format a value but it does not retrieve the endpoint attribute on its own.

!FindInMap reads static values from the template Mappings section and does not access attributes created at resource creation time. It is for template lookups and not for fetching resource endpoints.

!Ref returns a resource logical name or a simple identifier and for RDS it typically yields the DB instance identifier rather than the connection endpoint attribute that consumers need.

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