Which technique was primarily used to determine the structure of DNA?

The technique primarily used to determine the structure of DNA is X-ray crystallography. This method was vital in elucidating the double helix structure of DNA. X-ray crystallography works by diffracting X-ray beams through crystallized samples of a molecule, allowing the determination of the arrangement of atoms within the crystal. By analyzing the diffraction patterns, researchers can build a 3D model of the molecule.

In the case of DNA, this technique was famously employed by James Watson and Francis Crick, who relied heavily on the X-ray diffraction images produced by Rosalind Franklin. These images demonstrated the helical structure of DNA and provided key measurements that informed the models they constructed. While Franklin's work provided critical insight through her own use of X-ray diffraction, the general category of X-ray crystallography encompasses the broader applications that led to the understanding of DNA's structure.

Therefore, the use of X-ray crystallography stands as the defining technique in this discovery, making it the correct choice in identifying how the structure of DNA was determined.