A male patient 30 years old with visual acuity of 6/6 in both eyes. Twelve hours ago he presented with drop of vision of the left eye. On examination, visual acuity was 6/6 in the right eye and 6/60 in the left eye. Fundus examination showed blurred edges of the left optic disc. The most probable diagnosis is: a. Raised intra cranial pressure b. Raised ocular tension c. Central retinal artery occlusion d. Optic neuritis