Open "Settings" once again, then go to System > Display > Advanced display settings, which is located under the Multiple displays section. You'll see a drop-down menu at the top, which, when clicked, ...

With the app open, go to the Dock and right-click on the Activity Monitor icon. 2. In the pop-up menu, move the cursor to Dock Icon. 3. Another pop-up menu appears with the monitors you can select.