Final answer:
Molecules that pass through a cell membrane by simple diffusion must be small and/or lipophilic, such as oxygen and carbon dioxide. Large polar or ionic molecules typically cannot cross without the aid of transport proteins due to the selective permeability of the phospholipid bilayer.
Step-by-step explanation:
Molecules must possess certain properties to pass through a cell membrane by simple diffusion. The correct choice is that they must be small and/or lipophilic (B). This is because the cell membrane's phospholipid bilayer is selectively permeable, allowing only certain types of molecules to diffuse through it unassisted.
For simple diffusion to occur, molecules generally need to be either small nonpolar substances, such as oxygen and carbon dioxide, or hydrophobic molecules like lipids. These can pass through the cell membrane down their concentration gradient without the need for energy. Large polar or ionic molecules, which are hydrophilic, are typically unable to cross the membrane easily due to their size and the repulsion from the hydrophobic tails inside the phospholipid bilayer. Instead, polar molecules and ions typically require special transport proteins or channels to help them cross the membrane, a process known as facilitated diffusion.