Final answer:
A nonpolar molecule such as a hydrocarbon is most likely to pass through the plasma membrane without the help of a transport protein, as it can easily diffuse through the phospholipid bilayer.
Step-by-step explanation:
The substances most likely to pass through the plasma membrane without the help of a transport protein are nonpolar molecules, such as hydrocarbons. The plasma membrane is composed of a phospholipid bilayer, which allows nonpolar molecules to diffuse through it more easily due to their compatibility with the hydrophobic tails of the lipids. In contrast, polar molecules, ions, and large molecules struggle to pass through the hydrophobic core of the membrane without assistance from transport proteins, channels, or carriers.