In what way would you restrict the assigned to field on a project task to only allow users with resource allocations to be assigned?

ServiceNow provides a specific project preference to control the available assignees on project tasks. By selecting the “Derive assignee list from resource plan” checkbox on the Preferences tab of a project form, the system automatically filters the 'Assigned to' field on all associated project tasks. This ensures that only users who have been formally allocated to the project via a confirmed or allocated resource plan can be assigned tasks, enforcing proper resource management.

A. Create an Access Control List (ACL) rule: ACLs control permissions to read, write, or create records/fields, not filter the available choices within a reference field lookup.
C. The system automatically does this and can only be disabled by an admin: This behavior is not automatic; it is an optional preference that must be explicitly enabled on a per-project basis.
D. Create a reference qualifier on the ‘assigned to’ field: While technically possible with custom scripting, it is not the standard, out-of-the-box method. The platform provides a specific preference for this exact purpose.

- ServiceNow Product Documentation, "Manage project preferences". Derive assignee list from resource plan: If you select this check box, the choice list for the Assigned to field on the Project Task form shows only the resources that are assigned to the project.