The correct answer is B. Storing third-party service endpoints in API Gateway stage variables that correspond to the stage is a best practice. Stage variables are key-value pairs that you can use to customize the behavior of a specific deployment stage. By using stage variables, you can manage different endpoints for development, user acceptance testing, and production environments easily and efficiently. This is more suitable than the other options, which either do not address the need for stage-specific configurations (A, C) or introduce unnecessary complexity (D).