Final answer:
The passive process by which a fat-soluble molecule moves from a high to a low concentration through the plasma membrane is known as simple diffusion (option A).
Step-by-step explanation:
The passive process in which a fat-soluble molecule moves through the plasma membrane from an area of high concentration to an area of low concentration is called simple diffusion. Passive transport of molecules, such as fat-soluble substances, does not require energy. Rather, it relies on the concentration gradient to move substances down from high to low concentration areas. This process is a form of potential energy dissipation and is essential for substances that can move freely across the cellular membrane's hydrophobic phospholipid bilayer.