**Core Concept**
The question is testing the ability to identify a diagnostic method for Neisseria meningitidis carriers. Neisseria meningitidis is a bacterium that can cause meningococcal disease, a severe infection that can be life-threatening. Carriers of this bacterium can be asymptomatic but still transmit the infection to others.

**Why the Correct Answer is Right**
The correct answer is the Quellung reaction, a serological test used to detect the presence of group-specific polysaccharide capsules in Neisseria meningitidis. This test is based on the principle of agglutination, where antibodies against the capsule react with the bacteria to form visible clumps. The Quellung reaction is a specific and sensitive method for identifying carriers of Neisseria meningitidis. The test is particularly useful in public health settings, such as in outbreaks or during mass immunization campaigns.

**Why Each Wrong Option is Incorrect**
* **Option A:** Blood culture is not a specific method for detecting carriers of Neisseria meningitidis, as it may not yield the bacteria even in carriers.
* **Option B:** Urine culture may be positive in some cases of Neisseria meningitidis infection, but it is not a reliable method for detecting carriers.
* **Option D:** PCR (Polymerase Chain Reaction) is a molecular diagnostic technique that can detect the presence of Neisseria meningitidis DNA, but it is not the most commonly used method for detecting carriers.

**Clinical Pearl / High-Yield Fact**
The Quellung reaction is a classic example of a serological test used in public health to detect carriers of infectious diseases. It is a valuable tool in the prevention and control of meningococcal disease outbreaks.

**Correct Answer:** C.