Final answer:
The molecule unlikely to diffuse across a synthetic lipid bilayer is a large hydrophilic molecule because its size and polar nature are incompatible with the hydrophobic core of the phospholipid bilayer.
Step-by-step explanation:
The molecule that is unlikely to diffuse across a synthetic lipid bilayer without protein components is D. Large hydrophilic molecule.
This is because large polar or ionic (hydrophilic) molecules cannot easily pass through the hydrophobic core of the phospholipid bilayer; they are generally excluded due to their size and polar nature.
Small hydrophobic molecules, on the other hand, can pass through the membrane more easily because they are compatible with the hydrophobic interior.
The synthetic bilayer mimics the plasma membrane's behavior, which selectively allows substances through based on size and polarity.