#color(blue)"Cholesterol"# is a precursor to many organic molecules such as estrogen, testosterone, Vitamin D, and bile acids. Cholesterol is also a major component in the lipid bilayer which serves to provide rigidity and fluidity. Because of these reasons, cholesterol is most likely the one which will be seen to pass the membrane.
#color(red)"B) Phospholipid (WRONG)"#: Well, the lipid bilayer is composed of many phospholipids, with the hydrophobic regions clustered together and the polar hydrophilic ends facing the interior of the cell and the extracellular fluid. You will most likely not see any free floating phospholipids passing through.
#color(red)"C) Integral protein (WRONG)"#: This is a type of membrane protein embedded in the plasma membrane of the cell. It can function as a transporter, receptor, channel, etc. One example would be the #"G protein"# involved in signal transduction pathways. These will not be passing through either as they are firmly anchored in the membrane for reasons mentioned here.
#color(red)"D) Peripheral protein (WRONG)"#: This is also another type of membrane protein which is found loosely attached to the hydrophilic side of the plasma membrane. Their functions mainly include cell communication. One example is the #"F"_1# portion of the #"ATPase protein"# involved in the electron transport chain.
