For example, in the picture above, there are three of them.
How should I refer to them?
As in a sentence like:
I excused myself and went to the toilet, but all of the [...] were occupied.
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These are called stalls. Wikipedia shows the definition of "stall" as
a small enclosure of some kind, usually less enclosed than a room.
and gives these examples:
The individual areas are called cubicles (BrE).