## **Core Concept**
The study of molecular structure is a critical aspect of understanding the physical and chemical properties of molecules. This field involves analyzing the three-dimensional arrangement of atoms within a molecule. Various techniques and disciplines contribute to this understanding.

## **Why the Correct Answer is Right**
The correct answer, **X-ray crystallography**, is a technique used to determine the three-dimensional structures of molecules, including proteins and nucleic acids. It involves scattering X-rays off electrons in a crystallized molecule, producing a diffraction pattern that can be used to determine the molecular structure. This technique has been pivotal in understanding the structure-function relationships of biological macromolecules.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **Nuclear Magnetic Resonance (NMR) spectroscopy** is indeed a technique used to study molecular structures, especially for smaller molecules or specific domains of larger molecules in solution, it is not the only method and might not provide the same level of detail as X-ray crystallography for larger, more complex structures.
- **Option B:** **Mass Spectrometry** is primarily used for determining the mass-to-charge ratio of ions and is not directly used for studying the detailed molecular structure but rather for identifying and quantifying molecules based on their mass.
- **Option C:** This option seems to be missing content but based on the context, if it were another technique, it would be incorrect for similar reasons as A and B, depending on what it was.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **X-ray crystallography** has been instrumental in the field of structural biology, allowing for the detailed study of molecular structures which is crucial for understanding biological functions and designing drugs.

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