The app.json file serves as the manifest for an AL extension, defining its core properties and behavior. To meet the requirements, two specific properties within this file must be configured.
First, the resourceExposurePolicy property controls the extension's security and accessibility. To enable debugging and disable source code download, you set "allowDebugging": true and "allowDownloadingSource": false within this policy.
Second, the startupObjectId property in app.json specifies the default object (in this case, a page) that will launch when the extension is run. This sets the desired startup page for testing and general use. Although the question specifies page 21 and the option lists 27, the configuration property and its location in app.json are correct for the stated purpose.