**Core Concept**
The faint letter mark can be made visible using a technique that exploits the properties of silver halides in photographic emulsions. This is a fundamental principle in forensic science and photography.

**Why the Correct Answer is Right**
The correct answer is based on the concept of **silver halide development**. When exposed to light, the silver halides in photographic emulsions undergo a chemical change, allowing the faint letter mark to become visible. This process is similar to that used in traditional black-and-white photography. The faint letter mark can be made visible by treating the surface with a developer solution, which reacts with the silver halides to produce a visible image.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not relate to the process of making a faint letter mark visible.
**Option B:** This option is incorrect because it is not a technique used to make a faint letter mark visible.

**Clinical Pearl / High-Yield Fact**
In forensic science, the development of faint impressions can be critical in solving crimes. This technique can be used to reveal hidden or erased marks, such as fingerprints or footprints, on various surfaces.

**Correct Answer:** C. Silver nitrate solution