Picture rolling a thick slab where friction and the pass reduction are such that the top and bottom surface layers get squeezed and elongated much more than the middle of the slab. That mismatch pulls the central core into tension along the rolling direction. If the centerline already has some weakness, like porosity from casting or a segregation band, this tension is enough to tear it open right along that mid thickness plane. As the slab leaves the rolls the two halves bow apart because of the residual bending, giving the classic open mouthed look the defect is named after. So alligatoring means the slab has split along its centerline, option (A).