MSIX app attach is the correct application source because it is designed to dynamically deliver applications to user sessions. With app attach, an application packaged in an MSIX format is stored on a network share and attached to a session host only when a user logs in. The application is not installed on the base operating system image. This process makes the application available exclusively for the duration of the user's session and detaches it upon logoff, perfectly fulfilling the requirement. This method separates the application from the OS, simplifying image management and enabling on-demand application delivery.

A. start menu: This option is used for applications that are already permanently installed on the session host's base image and registered in the Start Menu.
C. file path: This option points to an executable file of an application that is permanently installed on the session host's file system, not one available only during a session.
D. FSLogix Profile Container: This is a user profile management solution used to persist user data and settings across sessions, not an application source for publishing RemoteApps.

1. Microsoft Learn | What is MSIX app attach?: "MSIX app attach is a way to deliver MSIX applications to both physical and virtual machines... When a user signs in, the MSIX packages you've assigned to them are mounted to their session. The applications in the packages are immediately available to them, appearing as if they were installed locally." This confirms the session-based availability.

2. Microsoft Learn | Add and manage MSIX app attach and app attach applications in Azure Virtual Desktop: In the "Add an MSIX package to a host pool" section, the process described involves attaching a VHD(X) containing the application to a session host, which is then made available to users in that session.

3. Microsoft Learn | Publish built-in apps in Azure Virtual Desktop: This document details the "Start menu" and "File path" application sources. It states, "If you select Start menu, the service will look for the application in the start menu of the session hosts... If you select File path, you'll need to provide the path to the application on the session hosts." This shows these methods rely on pre-installed applications.