The correct option is (a) Fwb =Btwfcd
To elaborate: Web buckling strength at support of simply supported plate girder is given by Fwb =Btwfcd , where B is length of stiff portion of bearing plus additional length given by dispersion at 45˚to the level of neutral axis, tw is thickness of web, fcd is allowable compressive stress corresponding to assumed web strut according to buckling curve c.