**Core Concept**
Hansen's disease, also known as leprosy, is a chronic bacterial infection caused by *Mycobacterium leprae*. The diagnosis of Hansen's disease can be made through various methods, including the slit skin smear test, which detects the presence of bacilli in the skin tissue.

**Why the Correct Answer is Right**
The slit skin smear test is considered positive when there are at least 10 bacilli per 100 high-power fields (HPF) in the skin tissue. This indicates an active infection, and the presence of a significant number of bacilli is crucial for the diagnosis. The bacilli are usually found in the dermal nerves and skin tissues, and their presence is indicative of the disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** 1 bacillus per 100 HPF is too low to be considered positive for the slit skin smear test.
* **Option B:** There is no specific number of bacilli mentioned in the literature for a positive slit skin smear test.
* **Option C:** 100 bacilli per 100 HPF would be an extremely high number and not a realistic expectation for a slit skin smear test.

**Clinical Pearl / High-Yield Fact**
The slit skin smear test is a traditional method for diagnosing Hansen's disease, but it has largely been replaced by more modern techniques such as skin biopsy and PCR.

**Correct Answer:** C. 10 bacilli per 100 HPF.