I would say that the line is not considered "straight" inside the corner.
It start to get straight just after the corner, when both inside and outside contour of the corner are finished.
The answer would be C, the inner distance should be at least 5cm.
But if this makes a big difference for your robot maybe you need to tune the line following algorithm?
The difference between A and C is not big, and may be considered "within the acceptable tolerances".
Fredrik Löfgren, TC and OC member 2016 / 2017