The incorrect sentence is:
C: "In the long history of the world, only a few generations has been granted the role of defending freedom in its hour of maximum danger."
The subject "a few generations" is plural, so the correct verb should be "have" instead of "has." The corrected sentence would read:
"In the long history of the world, only a few generations have been granted the role of defending freedom in its hour of maximum danger."
The correct option is (C): In the long history of the world, only a few generations has been granted the role of defending freedom in its hour of maximum danger.