**Core Concept**
The tertiary structure of a protein refers to its three-dimensional arrangement of amino acids in space, which is crucial for its function and stability. This structure is determined by the interactions between amino acids, including hydrogen bonds, ionic bonds, and hydrophobic interactions.

**Why the Correct Answer is Right**
The correct answer involves the use of X-ray crystallography to determine the tertiary structure of a protein. This technique involves bombarding a crystal of the protein with X-rays and measuring the diffraction pattern produced. By analyzing this pattern, researchers can infer the three-dimensional arrangement of the protein's atoms, including its amino acids. This information is essential for understanding how proteins bind to other molecules, interact with their environment, and perform their biological functions.

**Why Each Wrong Option is Incorrect**
* **Option A:** Spectrophotometry is a technique used to measure the absorption of light by molecules, but it is not typically used to determine protein structure.
* **Option B:** Gel electrophoresis is a method used to separate proteins based on their size and charge, but it does not provide information about their three-dimensional structure.
* **Option C:** Mass spectrometry is a technique used to determine the molecular weight of proteins, but it does not provide information about their tertiary structure.

**Clinical Pearl / High-Yield Fact**
The tertiary structure of proteins is essential for their function, and changes in this structure can lead to disease. For example, mutations in the prion protein can cause prion diseases, such as Creutzfeldt-Jakob disease, by altering its tertiary structure and leading to misfolding.

**Correct Answer: C. X-ray crystallography**