**Core Concept**
The amino acid sequence of Cytochrome C, a crucial component of the mitochondrial electron transport chain, exhibits significant variations across different species. This phenomenon is attributed to the high degree of conservation of the protein's three-dimensional structure, which is essential for its function. The variations in amino acid sequence are primarily located in surface-exposed regions, which are less critical for the protein's activity.

**Why the Correct Answer is Right**
The variations in the amino acid sequence of Cytochrome C are mainly found in the surface-exposed regions, which are not directly involved in the protein's catalytic activity. These regions are more susceptible to mutations, allowing for the adaptation of the protein to different environments and species. The conserved regions, on the other hand, are crucial for maintaining the protein's structure and function.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect as the variations are not primarily found in the regions involved in substrate binding or catalysis.
* **Option B:** This option is incorrect as the variations are not equally distributed throughout the protein sequence.
* **Option C:** This option is incorrect as the variations are not solely due to genetic drift or random mutations.

**Clinical Pearl / High-Yield Fact**
The conservation of the three-dimensional structure of Cytochrome C highlights the importance of structural biology in understanding protein function and evolution.

**Correct Answer: C. surface-exposed regions**