The fluid mosaic model describes the

structure of the plasma membrane

Explanation:

Fluid Mosaic Model:

The fluid mosaic model, proposed by S.J. Singer and Garth L. Nicolson in 1972,explains the structure and composition of the cell membrane.The plasma membrane is a combination of phospoholipids, cholesterol, proteins and carbohydrates. According to the model, the plasma membrane is a lipid bilayer made of phospholipids. The bilayer contains a hydophilic end, which interacts with the extracellular matrix and the cytoplasm, and a hydrophobic core that contains phospholipid tails. The hydrophobic tails are sandwiched between the hydrophilic phospholipid heads.Cholesterol, although less in composition than phospholipids, is an important component of the plasma membrane that provides it its characteristic fluidity and elasticity.Proteins are abundantly scattered in the cell membrane. Most of these are transport proteins and transmembrane channels involved in cellular transport.

The correct answer is  b)