What are extensions of the plasma membrane that serve primarily to increase a cell's surface area called?

Answer is microvilli.

Explanation:

Plasma membrane possesses or comprises of channels, made up of proteins, and form openings for certain molecules to diffuse through and into the cell.

The major function of the microvilli is that it increases the surface area of the cell. This activity increases the rate of absorption, secretion and cellular adhesion.

It should be noted that, microvilli are projections or protrusions found on the cellular membrane.