This approach is cost-effective

Step Functions does not have a Definitions substitution property or feature, so that throws A out. The most cost effective has to be B

A is the correct answer.

https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-resource-stepfunctions-statemachine.html#:~:text=A%20map%20(string,key%2Dvalue%20map.

going with A .

This approach leverages CloudFormation's ability to dynamically substitute values within the definition of AWS resources. By using the DefinitionSubstitutions property for the AWS::StepFunctions::StateMachine resource, you can directly insert the API Gateway endpoint or other necessary parameters into the state machine's definition. This enables the state machine to reference the API Gateway API without hard-coding values, allowing for flexibility and reusability of the CloudFormation template across different deployments. It's also a cost-effective solution because it uses native CloudFormation capabilities without the need for additional resources or services.

The other options (B, C, and D) involve using additional resources or services that are not necessary for this requirement and would therefore be less cost-effective.