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.