## Core Concept
The question pertains to forensic medicine and the detection of bloodstains. **Luminol test** is a chemical test used for detecting the presence of blood. It reacts with the hemoglobin in blood to produce a blue-green luminescence.

## Why the Correct Answer is Right
The correct answer, **Luminol test**, is used for demonstrating old, washed-up bloodstains because it can detect even tiny amounts of blood that may not be visible to the naked eye. This test works by spraying a solution of luminol and hydrogen peroxide onto the suspected area. If blood is present, the luminol reacts with the hemoglobin in the blood, producing a chemiluminescent reaction that emits light, making the blood visible in a darkened room. This reaction occurs because **hemoglobin** acts as a catalyst for the oxidation of luminol by hydrogen peroxide.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not specify a recognized method for detecting bloodstains.
- **Option B:** This option is incorrect as it does not correspond to a known technique for visualizing bloodstains.
- **Option D:** This option is incorrect because it does not represent a valid method for detecting old, washed-up bloodstains.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the **Luminol test** can detect blood even after it has been cleaned or washed, making it a valuable tool in forensic investigations. However, it's essential to note that this test can produce **false positives** with other substances that contain peroxidases or can catalyze the luminol reaction.

## Correct Answer Line
**Correct Answer: C. Luminol test**