**Core Concept**
The tertiary structure of a protein refers to its three-dimensional arrangement of atoms in space, which is essential for its proper functioning. This structure is formed by the interactions between amino acids, including hydrogen bonding, ionic bonds, and hydrophobic interactions. Understanding the tertiary structure of proteins is crucial in biochemistry, as it affects their enzymatic activity, binding sites, and overall function.

**Why the Correct Answer is Right**
The correct answer is X-ray crystallography because it is a technique used to determine the three-dimensional structure of molecules, including proteins. In X-ray crystallography, a beam of X-rays is scattered by the atoms within the protein crystal, producing a diffraction pattern that can be used to reconstruct the protein's tertiary structure. This technique has been instrumental in determining the structures of numerous proteins, including enzymes, receptors, and antibodies. The data obtained from X-ray crystallography provides detailed information about the protein's secondary structure, including alpha helices and beta sheets, as well as its overall tertiary structure.

**Why Each Wrong Option is Incorrect**
**Option B:** Spectrophotometry is a technique used to measure the absorption of light by a molecule, which can provide information about its concentration, but it is not capable of determining the three-dimensional structure of a protein.
**Option C:** Electrophoresis is a technique used to separate molecules based on their size and charge, but it does not provide information about the tertiary structure of proteins.
**Option D:** Chromatography is a technique used to separate and purify molecules based on their interactions with a stationary phase, but it is not capable of determining the three-dimensional structure of proteins.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that X-ray crystallography is a crucial tool in structural biology, and its data have been instrumental in understanding the mechanisms of many enzymes and receptors. This knowledge has led to the development of numerous drugs and therapies.

**✓ Correct Answer: A. X-ray Crystallography**