**Core Concept**
The definition of a unit of diphtheria antitoxin is based on the concept of neutralization, where a specific amount of antitoxin is required to prevent the toxic effects of diphtheria toxin. This is a fundamental principle in immunology and pharmacology.

**Why the Correct Answer is Right**
The correct answer refers to the specific amount of diphtheria antitoxin required to neutralize a standard amount of diphtheria toxin. In 1923, the International Unit (IU) was defined as the amount of diphtheria antitoxin that neutralizes 100 units of diphtheria toxin in a specific amount of serum. This definition is based on the principle of neutralization, where the antitoxin binds to the toxin, preventing its toxic effects. The diphtheria toxin is a potent protein toxin that causes tissue damage and death by inhibiting protein synthesis.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately reflect the definition of a unit of diphtheria antitoxin. The definition is based on the neutralization of diphtheria toxin, not the inhibition of a specific disease process.
* **Option B:** This option is incorrect because it is not relevant to the definition of a unit of diphtheria antitoxin. The definition is based on the neutralization of diphtheria toxin, not the treatment of diphtheria disease.
* **Option C:** This option is incorrect because it is not a specific or accurate description of the definition of a unit of diphtheria antitoxin.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that the definition of a unit of diphtheria antitoxin is based on the neutralization of diphtheria toxin, and this concept is fundamental to understanding the immunology and pharmacology of diphtheria antitoxin.

**Correct Answer: C. 30 LD50 of diphtheria toxin.**