Final answer:
The least likely molecule to cross a lipid bilayer is ATP.
Step-by-step explanation:
The least likely molecule to cross a lipid bilayer would be ATP. The lipid bilayer of a cell membrane is selectively permeable, allowing certain molecules to pass through easily while restricting others. Small, nonpolar molecules such as O2, CO2, and N2 can freely cross the lipid bilayer through simple diffusion due to their nonpolar nature. H2O (water) can also move across the membrane through a process called osmosis. However, ATP is a large and polar molecule, and it cannot pass through the lipid bilayer without the assistance of proteins that facilitate its transport across the membrane.