**Core Concept**
Metachromatic granules are structures found in certain bacteria, such as diphtheria bacilli, and are composed of **polyphosphate**. These granules can be stained using specific dyes.

**Why the Correct Answer is Right**
The correct answer is related to the staining properties of metachromatic granules. **Toluidine blue** and **methylene blue** are commonly used to stain these granules, but the most specific stain is **Albert's stain** or **Neisser's stain**, however, the question is incomplete, generally, **methylene blue** or similar stains are used to demonstrate metachromatic granules, as they have an affinity for **polyphosphate**.

**Why Each Wrong Option is Incorrect**
**Option A:** Without the specific option text, we cannot determine why it is incorrect, but generally, not all stains are suitable for metachromatic granules.
**Option B:** Similarly, without the text, we cannot assess its correctness.
**Option C:** This option is also unknown.
**Option D:** The same issue applies here.

**Clinical Pearl / High-Yield Fact**
Metachromatic granules are a key feature for identifying certain bacteria, such as *Corynebacterium diphtheriae*, and their staining is an important diagnostic tool.

**Correct Answer:** D. Methylene blue