The correct answer is: The grey lines are bent.
The Hering illusion is an optical illusion in which the grey lines appear to be bent, even though they are perfectly straight. This illusion is caused by the way that our brains interpret the visual information from our eyes.
The vertical lines at the ends of the illusion create a strong sense of verticality. This makes our brains assume that the grey lines are also vertical, even though they are slightly tilted. This creates the illusion that the grey lines are bent.
To see the illusion more clearly, try covering up the vertical lines at the ends of the image. You will see that the grey lines now appear to be perfectly straight.
This illusion is a reminder that our brains are not always perfect at interpreting the world around us. Sometimes, our brains can be fooled by simple visual cues.