What are Singer and Nicolson known for?

The most recent model of plasma membrane is fluid-mosaic model which was proposed by Singer and Nicolson in 1972. According to this model, the membrane does not have a uniform disposition of lipids and proteins but is instead a mosaic of the two. Further, the membrane is not solid but is quasifluid. So, the correct answer is option B (Fluid-mosaic model of plasma membrane).