In which case a ray of …

Please note that it is optical centre and not pole for a lens.

There is no such case according to my knowledge.A ray of light parallel to principal axis is the one coming from infinity and hence after "refraction" it will definitely pass through the focus of the lens forming a real image.If in case the parallel ray has to pass through the optical centre(there is no refraction or deviation while passing through pole),it has to suffer other refractions and then pass finally through the pole rather than coming directly. •Or you can think logically.A ray parallel to principal axis,if passes through the optical centre never becomes parallel to principal axis as the optical centre itself is the centre of lens which also lies on principal axis and parallel rays never meet(here,ray of light and principal acis are parallel and hence they never neet).Therefore no such case is possible.