**Core Concept:** Acridine orange is a fluorescent dye used for staining DNA and RNA in fixed cells. It has differential staining properties for nucleic acids, staining acidic (rich in guanine and cytosine) DNA in a bright green color and neutral (rich in adenine and thymine) RNA in a red color. This characteristic property helps in distinguishing between DNA and RNA in cells.

**Why the Correct Answer is Right:** Acridine orange is a cationic dye that enters cells and binds to nucleic acids. In its neutral form, it intercalates into DNA, causing an increase in fluorescence. Upon binding to RNA, it forms a complex that does not cause a significant fluorescence increase. Therefore, the correct answer is:

**Correct Answer: C. RNA**

**Why Each Wrong Option is Incorrect:**

A. **Acridine orange:** This is another fluorescent dye with similar properties, but it does not differentiate between DNA and RNA.
B. **Calcium ion:** This is a cationic ion and not a fluorescent dye, so it is incorrect.
D. **Agarose gel electrophoresis:** This is a technique used to separate DNA and RNA based on their molecular weight, not a dye that binds to nucleic acids.

**Clinical Pearl:** The ability to differentiate between DNA and RNA using acridine orange staining is useful in identifying the cellular component responsible for a particular gene expression or genetic disorder. It is particularly relevant in genetic research, molecular biology, and in the study of RNA viruses, where discrimination of RNA and DNA is essential.