**Core Concept**
The fixing solution in X-ray processing is a critical component that prevents the reversal of the silver halide crystals, essentially "fixing" the image. This process involves the removal of any remaining light-sensitive silver halides.

**Why the Correct Answer is Right**
The fixing solution typically contains **thiosulfate**, which is a strong reducing agent that selectively removes the silver halides (silver bromide and silver iodide) from the photographic emulsion, thereby preventing further exposure to light and ensuring the stability of the developed image. This process also involves the reduction of silver ions to metallic silver, which is then removed from the emulsion. The thiosulfate ions (S2O3^2-) act as a reducing agent, donating electrons to reduce the silver ions to metallic silver, which is then washed out of the emulsion.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not mention a specific chemical component commonly used in the fixing solution.
* **Option B:** This option is incorrect because it is not a known ingredient in the fixing solution of X-ray processing.
* **Option C:** This option is incorrect because it refers to a different component used in the photographic process, not the fixing solution.

**Clinical Pearl / High-Yield Fact**
The fixing solution is a critical step in X-ray processing, as it prevents the reversal of the silver halide crystals, ensuring the stability and permanence of the developed image.

**Correct Answer: C. Thiosulfate**