Right option is (c) Littrow mounting
For explanation I would say: Quartz shows the property of double refraction. Therefore, two pieces of quartz, one right-handed and one left handed are taken and cemented back-to-back in the construction of 60° prism (Cornu mounting), or the energy must be reflected and returned through a single 30° prism, so that it passes through the prism in both directions (Littrow mounting).
