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If I understand your sketches correctly, "A" & "C" have return duct which starts at the ceiling return grille and runs all the way to the return fan. These are considered "fully ducted" and should use the 125Pa additional pressure rise for the return fan. "B" draws air from a plenum and does not qualify for the additional 125Pa.

Your exhaust fan configuration is novel, but the principal is that the extra duct from the main duct / shaft to the return grille requires more fan power, so "a" & "C" appear to be the correct systems for applying the adder.

Note the exception: if the return system is a combination of fully ducted and plenum return, the adder may not be used.