That "Tool Options" window you have open contains one tab, which is (also) called "Tool Options". You can drag it (dragging the tab, not the window that it is inside of) on to any of the other tabs representing docked dialogs and the window will disappear and Tool Options will be docked as a tab there. By default the Tool Options dialog would be docked in the top right region where you currently have four docked dialogs (with Brushes selected). HTH.