Answer: Deploy the application as AWS Lambda functions. Set up Amazon API Gateway REST API endpoints for the application. Set up an Amazon Cognito user pool, and configure an Amazon Cognito authorizer.

The chosen answer (C) is the MOST operationally efficient solution that meets the requirements for the following reasons: 1. **AWS Lambda Functions**: Deploying the application as AWS Lambda functions means you are leveraging a serverless compute service where you do not need to manage any infrastructure or servers. AWS handles the scaling, patching, and other infrastructure management tasks, thus reducing operational overhead. 2. **Amazon API Gateway**: By setting up Amazon API Gateway REST API endpoints, you get a fully managed service that makes it easy to create, publish, maintain, monitor, and secure APIs at any scale. Like AWS Lambda, it offloads the burden of managing infrastructure. 3. **Amazon Cognito User Pool**: Setting up an Amazon Cognito user pool provides a scalable and secure identity store for managing user authentication and access control. Amazon Cognito can handle tasks related to user registration, authentication, and account recovery without requiring the IT team to manage any server infrastructure. 4. **Cognito Authorizer with API Gateway**: Configuring Amazon API Gateway to use an Amazon Cognito authorizer allows you to securely control access to your APIs. The integration between Amazon API Gateway and Amazon Cognito is smooth and leverages native AWS features, ensuring streamlined operations and management. Overall, this combination provides a robust, scalable, and fully managed environment for deploying a REST API application without the need for maintaining any underlying infrastructure, making it the most operationally efficient solution.

Author: LeetQuiz Editorial Team