Which out of following, cannot produce two coherent sources?

Lloyd's mirror is used to produce to two coherent sources out of phase with each other, from a single source and its reflection form a mirror.
Fresnel biprism refracts light from a single source to create two virtual in-phase coherent sources.
Young's double slit produces two coherent sources by diffraction from two slits, phase depending upon the position of the source with respect to the slits.
However a single prism cannot produce two coherent sources.