**Core Concept**
Passive transport is the movement of molecules across a cell membrane without the need for energy expenditure. It involves the movement of substances down their concentration gradient, from an area of higher concentration to an area of lower concentration. Neutral substances, such as water and ions, can be transported across the cell membrane through various mechanisms.

**Why the Correct Answer is Right**
The correct mechanism for the transport of neutral substances is diffusion. Diffusion is the process by which molecules move from an area of higher concentration to an area of lower concentration, resulting in uniform distribution. This process is driven by the random motion of molecules and does not require energy expenditure. In the case of neutral substances, diffusion can occur through the lipid bilayer of the cell membrane, as the molecules are not charged and can easily pass through the non-polar region of the membrane.

**Why Each Wrong Option is Incorrect**
**Option A:** Osmosis is the movement of water molecules across a semipermeable membrane from an area of higher concentration to an area of lower concentration. While osmosis is a type of passive transport, it specifically refers to the movement of water, not neutral substances in general.

**Option B:** Active transport is the movement of molecules across a cell membrane against their concentration gradient, requiring energy expenditure. This is the opposite of the correct mechanism, as neutral substances are transported down their concentration gradient through passive transport.

**Option C:** Facilitated diffusion is the movement of molecules across a cell membrane with the assistance of transport proteins. While this can occur with neutral substances, it is not the primary mechanism for their transport across the cell membrane.

**Clinical Pearl / High-Yield Fact**
Remember the phrase "Solute, solvent, and semipermeable membrane" to recall the key components involved in osmosis, a type of passive transport.

**Correct Answer: D. Diffusion**