Refer to the exhibit. A developer needs to create a Docker image that listens on port 5000. Which code snippet must be placed onto the blank in the code?

The EXPOSE instruction informs Docker that the container listens on the specified network ports at runtime. While it does not actually map the port to the host (which is done during docker run using the -p or -P flags), it serves as a crucial piece of metadata for both documentation and automated mapping. According to official Docker documentation, EXPOSE is the standard instruction used within a Dockerfile to define which ports the application inside the container is intended to use for communication.

A. PORT 5000: This is not a valid Dockerfile instruction. While "PORT" is often used as an environment variable in application code, it has no native function in the Dockerfile syntax.

B. LISTEN 5000: This is not a valid Dockerfile instruction. "Listen" is a common directive in web server configurations (like Nginx or Apache) but is not part of the Dockerfile specification.

D. OPEN 5000: This is not a valid Dockerfile instruction. There is no "OPEN" command in the Docker engine's build syntax to manage network ports.

Official Docker Documentation: "Dockerfile reference: EXPOSE." [Link: docs.docker.com/engine/reference/builder/#expose]

Docker Curriculum (University/Open Source): "Dockerizing a Python Web App - The EXPOSE Instruction." [Link: docker-curriculum.com]

Harvard CS50's Introduction to Computer Science: "Cloud Computing and Docker - Network Port Mapping and EXPOSE." [Reference: Lecture 9, Section: Containerization].