**Core Concept**
The question is testing the concept of equilibrium in the context of blood alcohol concentration (BAC) and its relation to the concentration of alcohol in the stomach, intestines, and bloodstream. The underlying principle is the concept of diffusion and the rate of absorption of alcohol from the stomach and intestines into the bloodstream.

**Why the Correct Answer is Right**
The correct answer is related to the concept that the stomach has a larger surface area for absorption compared to the intestines, and the stomach is more permeable to alcohol due to the presence of mucous membranes. When alcohol is ingested, it is absorbed rapidly from the stomach into the bloodstream through passive diffusion. As the stomach empties into the small intestine, the rate of absorption slows down due to the smaller surface area and lower permeability of the intestinal mucosa. Therefore, the stomach will have the highest relative concentration of alcohol at equilibrium.

**Why Each Wrong Option is Incorrect**
* **Option A:** The liver is responsible for metabolizing alcohol, but it is not the site of highest concentration at equilibrium. The liver will have a higher concentration of alcohol metabolites, but not the highest concentration of alcohol itself.
* **Option B:** The intestines will have a lower concentration of alcohol compared to the stomach, as the rate of absorption is slower due to the smaller surface area and lower permeability.
* **Option C:** The bloodstream will have a lower concentration of alcohol compared to the stomach, as the rate of absorption is slower and there is a dilution effect from the blood volume.

**Clinical Pearl / High-Yield Fact**
The stomach is responsible for approximately 20-30% of total alcohol absorption, while the small intestine accounts for the remaining 70-80%. The rate of absorption is influenced by factors such as gastric emptying, food intake, and individual tolerance.

**Correct Answer:** C. The intestines.