Which substance would be most likely to pass through a lipid bilayer/plasma membrane without embedded proteins?

Answer; The correct answer is-

Small polar and hydrophobic molecules can pass through a lipid bilayer / plasma membrane.

Plasma membrane (surrounding the cytoplasm of cells) is a selectively permeable membrane that is primarily composed of two layer of phospholipids (hydrophobic in nature) and regulates the entry and exit of molecules in and out of the cell.

As it is hydrophobic in nature, so it does not allow large polar molecules (such as proteins) and ions to cross it.

Thus, only small polar molecules and hydrophobic (that is non polar) molecules can pass through it.

Example - Water, ethanol, oxygen, carbon dioxide