RESTful web APIs are architecturally constrained to the client-server model and communicate over the HTTP protocol. When security is a requirement, as implied by the use of Anypoint Flex Gateway, the secure version, HTTPS (HTTP Secure), is used. Application Load Balancers (ALBs) operate at the application layer (Layer 7) and are specifically designed to inspect and route HTTP/HTTPS traffic based on request content. Therefore, the ALB uses the HTTPS protocol to route requests to the RESTful API fronted by the Flex Gateway, ensuring secure, managed communication.

B. SMTP: Simple Mail Transfer Protocol (SMTP) is used exclusively for sending and receiving email, not for invoking RESTful web APIs.

C. LDAP: Lightweight Directory Access Protocol (LDAP) is used for querying and managing information in a directory service, not for API request-response patterns.

D. SFTP: SSH File Transfer Protocol (SFTP) is a network protocol used for secure file access, transfer, and management over a reliable data stream.

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1. MuleSoft Official Documentation, "HTTP Listener": The HTTP Listener is the component used to trigger a Mule flow when a request is received at a specific URL. It is the primary entry point for RESTful APIs built in MuleSoft, confirming the foundational use of HTTP/HTTPS.

Source: MuleSoft Docs, Connectors > HTTP Connector > HTTP Listener.

Section: "The HTTP Listener connector is an inbound endpoint that listens for HTTP requests on a specific host and port."

2. MuleSoft Official Documentation, "About Flex Gateway": This document describes Flex Gateway as an API gateway designed to manage and secure APIs. API gateways inherently operate on API traffic, which for RESTful services is HTTP/HTTPS.

Source: MuleSoft Docs, Anypoint Flex Gateway > About Flex Gateway.

Section: "Anypoint Flex Gateway is an Envoy-based, ultrafast, and lightweight API gateway that you can deploy to manage and secure APIs running anywhere." (Envoy Proxy is a Layer 7 proxy for HTTP traffic).

3. MuleSoft Official Documentation, "Deploy Flex Gateway as a Docker Container": Deployment guides for Flex Gateway explicitly detail the configuration of ports for HTTP and HTTPS traffic, reinforcing that these are the protocols it is designed to handle.

Source: MuleSoft Docs, Anypoint Flex Gateway > Deploy Flex Gateway > Deploy Flex Gateway as a Docker Container.

Section: "Prerequisites" and configuration examples often show port mappings like -p 8081:8081, which is the default port for HTTP/HTTPS traffic handled by the gateway.