**Core Concept**
Henry's law describes the relationship between the partial pressure of a gas and its concentration in a solution, specifically in a liquid at equilibrium. This principle is crucial in pharmacology, physiology, and toxicology, as it helps predict the solubility of gases in blood and other tissues.

**Why the Correct Answer is Right**
Henry's law states that the concentration of a gas in a solution is directly proportional to its partial pressure above the solution. This means that as the partial pressure of a gas increases, its concentration in the solution also increases. The mathematical expression of Henry's law is C = kP, where C is the concentration of the gas, k is the Henry's law constant, and P is the partial pressure of the gas. This relationship is essential in understanding the behavior of gases in biological systems.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe Henry's law. The correct relationship between gas concentration and partial pressure is described by Henry's law, not this option.
**Option B:** This option is incorrect because it refers to another gas law, Graham's law of diffusion, which describes the rate of diffusion of gases in a mixture, not the relationship between gas concentration and partial pressure.
**Option C:** This option is incorrect because it refers to the behavior of gases in a mixture, not the relationship between gas concentration and partial pressure.

**Clinical Pearl / High-Yield Fact**
Henry's law is essential in understanding the behavior of gases in biological systems, particularly in the context of anesthesia, where the partial pressure of anesthetic gases is critical in determining their effect on the body.

**Correct Answer: C. Partial pressure of a gas is directly proportional to its concentration in a solution.**