Answer:
take your pick. Each has its own reason.
Step-by-step explanation:
As with a lot of "doesn't belong" problems, the one that is different depends on your criteria. In the attached figure, each one "doesn't belong" for a different reason. Those reasons are listed below each number line.
__
I might choose the lower left: "addition, not subtraction", as it is the only one with arrows in the same direction. That makes its difference from the others more obvious, perhaps.
