**Core Concept**
The question is testing the ability to identify a diagnostic method for Cryptococcus, a type of fungus that can cause serious infections, particularly in immunocompromised individuals. Cryptococcus is known to cause cryptococcal meningitis, a life-threatening condition.

**Why the Correct Answer is Right**
Cryptococcus can be readily demonstrated by India ink stain, also known as the India ink preparation or nigrosin stain. This method involves adding a drop of India ink to a wet mount of cerebrospinal fluid (CSF) or other bodily fluid and examining it under a microscope. The ink particles do not penetrate the capsules of the Cryptococcus cells, making them appear as a clear halo around the cells, allowing for easy identification.

**Why Each Wrong Option is Incorrect**
* **Option A:** Gram stain is not a suitable method for demonstrating Cryptococcus, as it does not stain the capsule and can be difficult to interpret.
* **Option B:** Acid-fast stain is typically used to identify Mycobacterium species, not Cryptococcus.
* **Option C:** Calcofluor white stain can be used to detect fungal elements, but it is not the most specific or sensitive method for demonstrating Cryptococcus.

**Clinical Pearl / High-Yield Fact**
Cryptococcal meningitis is a common opportunistic infection in HIV/AIDS patients, particularly those with low CD4 counts. India ink stain is a rapid and inexpensive method for diagnosing this condition, and it can be performed in resource-limited settings.

**Correct Answer: D. India ink stain**