## Core Concept
The question tests knowledge on the cultivability of various viruses. Virus cultivation is a critical aspect of virology, often requiring specific cell cultures or embryonated eggs. Some viruses are difficult or impossible to culture due to their fastidious growth requirements or the lack of suitable host cells.

## Why the Correct Answer is Right
Prions are infectious proteins that cause fatal neurodegenerative diseases in animals, including Creutzfeldt-Jakob disease in humans. Unlike conventional viruses, prions do not contain nucleic acids (DNA or RNA) and therefore cannot be cultivated using traditional virological cell culture techniques. Their unique nature makes them impossible to culture in the same way viruses are.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is not provided, but typically, many viruses can be cultivated using appropriate cell cultures or media.
* **Option B:** Similarly, without the specific content of option B, it's understood that some viruses are cultivable under specific conditions.
* **Option C:** This option is also not provided, but the implication is that some viruses can indeed be cultivated.

## Clinical Pearl / High-Yield Fact
A key point to remember is that prions are not viruses; they are misfolded proteins that can induce normal proteins in the brain to also misfold, leading to brain damage and the characteristic symptoms of prion diseases. This distinction is crucial for understanding their transmission, diagnosis, and the fact that they cannot be cultured like viruses.

## Correct Answer: D. Prions