**Core Concept**
The Schick test is a method used to determine an individual's susceptibility to diphtheria, a bacterial infection caused by Corynebacterium diphtheriae. This test assesses the presence of circulating antibodies against diphtheria toxin, which is produced by the bacteria.

**Why the Correct Answer is Right**
A positive Schick test result indicates the presence of circulating antibodies against diphtheria toxin, suggesting that the individual has been previously exposed to the bacteria or has received immunization against diphtheria. The presence of these antibodies signifies an immune response, but in this context, it is an indication of susceptibility to diphtheria rather than immunity. This is because the antibodies may not be sufficient to confer long-term immunity, and the individual may still be at risk of infection. The Schick test is often used to identify individuals who require vaccination against diphtheria.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a positive Schick test result does not indicate complete immunity to diphtheria. While the presence of antibodies may provide some protection, it is not a guarantee against infection.

**Option B:** This option is incorrect because hypersensitivity to diphtheria is not directly related to the outcome of the Schick test. The test assesses susceptibility to the bacteria, not an allergic response.

**Option D:** This option is incorrect because a carrier state is not indicated by a positive Schick test result. Carriers of diphtheria typically have a high level of circulating antibodies against the bacteria, which would result in a negative Schick test.

**Clinical Pearl / High-Yield Fact**
It's essential to note that a negative Schick test result does not guarantee lifelong immunity to diphtheria. Booster shots are often recommended to maintain immunity, especially for individuals who are at risk of exposure to the bacteria.

**✓ Correct Answer: C. Susceptible to diphtheria**