In order for a Question Bank to be used in Diagnostic Assessments, navigate to the Menu and click on the Question Banks link.
Locate the Question Bank you wish to amend and open it by clicking on its title.
In the Details tab, tick the Can use in Diagnostic Assessments box and click Save.
You will now be able to use the Question Bank when creating Diagnostic Assessments using Activities which are linked to Questions in the Question Bank. See the Complete the Question Bank Template guide for how to link a question to an Activity in the LMS or for help building a Diagnostic Assessment, see the Create a Diagnostic Assessment guide.