The correct answer is option 1: a bilayer of phospholipid molecules in which proteins and cholesterol are embedded.

The membrane is a crucial component of cells and is responsible for separating the internal contents of the cell from the external environment. It acts as a barrier that regulates the passage of substances in and out of the cell.

The membrane consists of a double layer of phospholipid molecules, which are amphipathic. This means that they have a hydrophilic (water-loving) head and a hydrophobic (water-repelling) tail. The hydrophilic heads face the aqueous environments, both inside and outside the cell, while the hydrophobic tails are oriented towards the interior of the membrane.

Embedded within the phospholipid bilayer are proteins. These proteins serve various functions, such as transporting molecules across the membrane, acting as receptors for signaling molecules, or providing structural support. Cholesterol molecules are also present in the membrane, contributing to its fluidity and stability.

Option 2, a bilayer of protein molecules in which lipid and cholesterol are embedded, is incorrect because proteins are embedded within the lipid bilayer, rather than the other way around.

Option 3, a bilayer of neutral lipid in