The correct answer is A, AWS Step Functions. This service is specifically designed for building distributed applications with a low-code, visual workflow approach. It allows developers to define and orchestrate the steps and conditions of their application's workflow visually, making it easier to manage complex processes. The other options, while important AWS services, do not match the requirements of the question: B (AWS Config) is for resource configuration monitoring, C (AWS Lambda) is for running code in response to events, and D (Amazon CloudWatch) is for monitoring AWS resources and applications.