This question is for finding the condition where system stability is maintained after the fault.
From equal angle criteria, for stability of system: area C = area B.
Since network restores at 80deg and the power equations are not given we assume this as critical clearing angle.
"Critical clearing angle states that If the fault is cleared before critical clearing angle (here 80deg), stability is maintained."
So for fault clearing angle less that 80 deg; area C will be less than area B and still stability is maintained.
Hence the condition is : area c <= area B; conv