In the detached 'Tool Options' window, click on the red 'X' in the upper right corner to get rid of the window. Then on the main Gimp screen, click on 'Windows,' then 'Dockable Dialogs.' The first entry on its list will be 'Tool Options,' so click on that. Then, Tool Options will appear as a tab in the window on the right side of the screen, along with layers and undo history. Click and drag that tab over to the toolbox window on hte left and drop it inside. The tool options will again be docked in the toolbox.