How can services be defined in a Docker Compose file?

In a Docker Compose file, services are defined using the keyword "services" followed by the service definitions. This structure allows users to specify multiple services that make up their application, each with its own configuration details. The service definition typically includes parameters such as the image to use, commands to run, ports to expose, and any environment variables specific to that service. This makes it clear and organized, facilitating the management of multi-container Docker applications.

The other options, while related to aspects of Docker and container configuration, do not fundamentally define the services themselves in a Docker Compose file. Specifying network settings, configuring environment variables, or defining volume mounts are tasks that can be performed within the context of defining a service but do not encapsulate how services are introduced or structured in the Compose file. Therefore, the use of the "services" keyword is crucial for defining which components or microservices are part of the application.