@glb, when we’re using the verbs ser or estar to indicate a location/position, they’re generally interchangeable. A third verb you could also use here is ficar. So:
O cérebro está na cabeça. = O cérebro é na cabeça. = O cérebro fica na cabeça.
By the way, you can always ask questions/report issues from within the units. All you need to do is click on the 3-dot menu to the right of the progress bar and select Report a Problem , before moving on to the next question. It will automatically send us information about that question (including the answer you gave/selected, so you don’t have to copy/paste it).