**Core Concept**
The investigation of choice for diphtheria carriers involves identifying individuals who harbor the bacterium Corynebacterium diphtheriae in their throat without exhibiting symptoms of the disease. This is crucial for controlling outbreaks and preventing transmission to others.

**Why the Correct Answer is Right**
The investigation of choice for diphtheria carriers is the **cervical culture**. A throat swab is taken from the individual, and the sample is cultured on a suitable medium, such as tellurite agar, to isolate and identify C. diphtheriae. This method is sensitive and specific, allowing for the detection of carriers who may not show symptoms of the disease.

**Why Each Wrong Option is Incorrect**
**Option A:** **Nasopharyngeal culture** is not the investigation of choice for diphtheria carriers. While nasopharyngeal swabs can be used to detect respiratory pathogens, they are not the preferred method for identifying diphtheria carriers.

**Option B:** **Blood culture** is not used to diagnose diphtheria carriers. Diphtheria is a bacterial infection caused by C. diphtheriae, but blood cultures are not the primary method for detecting carriers.

**Option C:** **Urine culture** is not relevant to the investigation of diphtheria carriers. Diphtheria is a respiratory infection, and urine cultures are not used to diagnose or detect carriers.

**Clinical Pearl / High-Yield Fact**
It's essential to note that diphtheria carriers can be asymptomatic for long periods, making regular screening and testing crucial in areas with high transmission rates.

**Correct Answer:** C.