Which pressure mode of operation has the most stress on aircraft frame So I am want to know which mode of pressurization has the most demand on the airframe 

Isobaric mode

constant - differential mode

unpressurized

I hope to get an answer because I have an exam and need to know
Thank you very much
Isobaric mode causes the most stress if you keep the cabin pressure and climb higher and higher. (Because the delta pressure and thus the forces become higher and higher)
That's why at high altitudes the isobaric mode is no longer used and the cabin pressurization switches to constant differential pressure instead, which limits the maximum stress.
And unpressurized, well that is not really stressing the hull at all