**Core Concept**
The underlying principle being tested is the diagnosis of cutaneous anthrax, caused by *Bacillus anthracis*, which can be occupationally acquired. The bacterium is typically found in animals and can be transmitted to humans through contact with infected animals or contaminated animal products.

**Why the Correct Answer is Right**
The correct answer is related to the identification of *Bacillus anthracis*. The diagnosis can be confirmed by staining the specimen with a specific method that highlights the characteristic appearance of the bacteria.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not the best method for diagnosing *Bacillus anthracis*.
**Option B:** This option is also incorrect as it does not specifically identify the bacteria.
**Option D:** Similarly, this option is incorrect as it is not the preferred staining method for this diagnosis.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that cutaneous anthrax typically presents as a painless, ulcerative lesion with a characteristic black eschar.

**Correct Answer:** D. McFadyean's reaction