To make a custom data endpoint from Business Central available to an external application like a canvas app, it must be published as a web service. While this can be done manually, the modern and repeatable approach is to automate this process within an AL extension. An Install codeunit is specifically designed to run code when an extension is installed. By calling the WebService.Publish function from within an Install codeunit, the page or query is automatically published as an OData web service upon the extension's installation, making the RoomsAPI endpoint accessible.

A. Use the default API configuration in Business Central This applies to the standard, built-in v2.0 APIs provided by Microsoft, not for publishing a custom page or query like RoomsAPI.
B. Enable the APIs for the Business Central online environment. This is a general setting, typically enabled by default, that governs the standard v2.0 APIs. It does not publish custom endpoints.
C. Open the Web Services page and publish the RoomsAPI page as a web service. This describes the manual process for publishing a web service. While functional, it is not an automated or scalable solution for deploying customizations, which is the focus of modern AL development.

1. Microsoft Learn | Dynamics 365 Business Central Documentation. "Publishing a Web Service". This document explicitly states, "You can publish a web service programmatically by using the WebService.Publish function. This lets you automate the process of publishing web services when an extension is installed." This directly supports the method described in option D.

2. Microsoft Learn | Dynamics 365 Business Central Documentation. "Install Codeunits". This document explains that codeunits with SubType = Install; are used to run code during the installation of an extension, which is the correct context for programmatically publishing a web service.

3. Microsoft Learn | Dynamics 365 Business Central Documentation. "WebService Data Type". This page details the Publish function: WebService.Publish(ObjectType: ObjectType, ObjectId: Integer, ServiceName: Text). This is the specific AL function used within the install codeunit to perform the action.