Your team plans to launch two CloudFormation stacks to create resources for a web application. These two stacks need to be managed separately. The first stack is a network stack that includes network resources such as subnets and security groups. The second stack is an application stack used to launch an Auto Scaling group resource. The ASG needs to refer to the resources created by the network stack. How would you configure the application stack to import the network resources properly?

Answer options:

A.Export the resources in the resource properties of the network stack. In the application stack, import the resources with the intrinsic function Fn::GetAttr.
B.Export the resources in the outputs of the network stack. In the application stack, import the resources with the intrinsic function Fn::Sub.
C.Export the resources in the outputs of the network stack. In the application stack, import the resources with the intrinsic function Fn::ImportValue.
D.In the network stack, export the resources through the intrinsic function Fn::ExportValue. In the application stack, import the resources with the intrinsic function Fn::FindInMap.

Correct Answer: C
Option A is incorrect because in the network stack, you should export the resources in the outputs of the stack. The following example exports the VPC ID:
 "Outputs" : {
"VPCId" : {
"Description" : "VPC ID",
"Value" :{ "Ref" : "VPC" },
"Export" : { "Name" : {"Fn::Sub": "${AWS::StackName}-VPCID" }}
}
}
Option B is incorrect because Fn::Sub is used to replace a string. To import values that have been exported, the intrinsic function Fn::ImportValue should be used.
Option C is CORRECT because the intrinsic function Fn::ImportValue can return the value of the outputs exported by the network stack and should be used in this scenario.
Option D is incorrect because there is no intrinsic function Fn::ExportValue. Besides, Fn::FindInMap cannot be used to create cross-stack references.
Reference:
https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/walkthrough-crossstackref.html