Final answer:
Small nonpolar molecules can easily cross the lipid bilayer without the aid of a transport protein, unlike ions and large polar molecules which need specialized transport proteins. Option 3 is the correct answer.
Step-by-step explanation:
The substances that would readily cross a lipid bilayer without the aid of a transport protein are small nonpolar molecules. These molecules can easily diffuse across the cell membrane due to their nonpolar nature, which is compatible with the hydrophobic interior of the lipid bilayer. On the other hand, ions and large polar molecules encounter resistance from the hydrophobic interior and require specialized transport proteins, such as channel proteins, gated channel proteins, or carrier proteins, to facilitate their movement across the membrane through a process known as facilitated diffusion.
Water molecules, despite being polar, can cross the membrane to a limited extent through simple diffusion but also utilize specialized channels called aquaporins for more efficient passage. Substances like oxygen and carbon dioxide are examples of nonpolar molecules that can pass freely through the membrane without the need for transport proteins, moving along their concentration gradients in a process known as passive transport.