A question pool consists of questions developed in advance and saved for use in assessments.
Select the Tests & Quizzes tool from the Tool Menu in your site.
You will see the question pool details, and links for adding subpools and questions. In the Questions section, click the Add Question link on the right hand side of the screen.
All of the standard question types are available in the drop-down menu. (See the help articles on individual question types for more information on adding specific types of questions to the pool.)