The lateral movement of transmembrane proteins can be restricted by several different mechanisms. Which mechanism best describes separating transmembrane proteins on the apical surface of the epithelial cells from their basal and lateral surfaces by limiting lateral movement of any transmembrane proteins?

(a) proteins are tethered to the cell cortex
(b) proteins are tethered to the extracellular matrix
(c) proteins are tethered to the proteins on the surface of another cell
(d) protein movement is limited by the presence of a diffusion barrier