**Core Concept**
Transport of neutral substances across the cell membrane primarily occurs through passive diffusion, a process driven by concentration gradients. This type of movement does not require energy and is facilitated by the cell membrane's lipid bilayer structure, allowing molecules to freely move from an area of higher concentration to an area of lower concentration.

**Why the Correct Answer is Right**
Passive diffusion is the primary mechanism for the transport of neutral substances across the cell membrane. This process relies on the cell membrane's lipid bilayer structure, which allows molecules to freely move through the membrane. The lipid bilayer's hydrophobic core creates a barrier that restricts the movement of charged particles or polar molecules, but neutral substances, such as oxygen, carbon dioxide, and certain hormones, can easily diffuse through the membrane.

**Why Each Wrong Option is Incorrect**
**Option A:** Porins are transmembrane proteins that facilitate the transport of polar molecules across the cell membrane, typically through channels or pores. They are not responsible for the transport of neutral substances.
**Option B:** A lonophore is a class of compounds that facilitate the transport of charged particles across the cell membrane through a process called facilitated diffusion. While lonophores can transport charged particles, they are not involved in the transport of neutral substances.
**Option C:** Lipopolysaccharides are large molecules found in the outer membrane of Gram-negative bacteria. They play a role in the bacterial cell's structural integrity and immune evasion but are not involved in the transport of neutral substances across the cell membrane.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the transport of neutral substances across the cell membrane is primarily driven by passive diffusion, which relies on concentration gradients. This process is distinct from active transport, which requires energy and is often driven by ATP hydrolysis.

**✓ Correct Answer: D. Diffusion**