## Core Concept
The question tests the understanding of capillary permeability across different organs. Capillary permeability varies significantly among tissues, influencing the exchange of substances between blood and tissues. This variation is crucial for the proper functioning of different organs.

## Why the Correct Answer is Right
The correct answer, **Liver**, has the most permeable capillaries. This high permeability is due to the unique structure of hepatic sinusoids, which are not true capillaries but rather large, fenestrated channels. These sinusoids allow for the free exchange of a wide range of substances, including proteins and even small particles, between hepatocytes and the bloodstream. This characteristic is essential for the liver's metabolic and detoxification functions.

## Why Each Wrong Option is Incorrect
- **Option A:** The brain has relatively impermeable capillaries, forming the blood-brain barrier, which restricts the passage of solutes and cells into the brain, protecting it from harmful substances.
- **Option B:** Muscle capillaries are moderately permeable, allowing for the exchange of oxygen, nutrients, and waste products but restricting the passage of larger molecules.
- **Option C:** Lung capillaries are highly permeable to gases (oxygen and carbon dioxide) but have limited permeability to larger molecules and particles, which is crucial for gas exchange.
- **Option D:** While some organs not listed might have high permeability, among the given options, liver stands out for its exceptionally high capillary permeability.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the liver's high capillary permeability facilitates its role in detoxification and metabolism. This characteristic allows for the efficient uptake and processing of substances by hepatocytes.

## Correct Answer: C. Liver