A slat if extended at a low angle of attack has very little effect on the CL of a wing ( slat increase CL max) but it would increase CD . However if you were to extend a trailing edge flap at the same value of alpha it would increase CL.

So if one flap was to extend and not the other there would be a large rolling moment produced. But if one slat was to remain extended there would be more drag on the wing causing the aircraft to yaw.

The examiner is trying to tell you by their explanation of what they mean by LARGE ( I agree it’s not very clear) that you would be unable to control the roll of asymmetric flap but the slat would increase CL max of the affected wing.