**Core Concept**
The tertiary structure of a protein refers to its unique three-dimensional shape, which is determined by the spatial arrangement of its amino acid residues. This structure is crucial for the proper functioning of proteins, including their enzymatic activity, binding properties, and interactions with other molecules. The tertiary structure is influenced by various factors, including hydrogen bonding, disulfide bridges, and hydrophobic interactions.

**Why the Correct Answer is Right**
The correct answer is **X-ray Crystallography**. This technique involves scattering X-rays off the atoms within a protein crystal, producing a diffraction pattern that can be used to determine the protein's three-dimensional structure. The resulting electron density map can be interpreted to reveal the protein's tertiary structure, including the positions of its amino acid residues and any ligands or cofactors bound to it. This information can be used to understand the protein's function and interactions with other molecules.

**Why Each Wrong Option is Incorrect**
* **Option A:** NMR Spectroscopy is a technique used to determine the secondary and tertiary structures of proteins, but it is not the most accurate method for determining the tertiary structure.
* **Option B:** Mass Spectrometry is a technique used to analyze the mass-to-charge ratio of ions, but it is not directly related to determining protein structure.
* **Option C:** Gel Electrophoresis is a technique used to separate proteins based on their size and charge, but it does not provide information about protein structure.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the tertiary structure of proteins is crucial for their proper functioning, and any changes to this structure can lead to protein misfolding and disease. For example, the misfolding of prion proteins has been linked to prion diseases such as Creutzfeldt-Jakob disease.

**Correct Answer: D. X-ray Crystallography**