The tool used to show the structure of DNA was X-ray diffraction.

Here's why:

* X-ray diffraction is a technique that uses X-rays to study the arrangement of atoms within a crystal or molecule.

* Rosalind Franklin, a scientist working at King's College London, used this technique to take the famous "Photo 51" of DNA.

* James Watson and Francis Crick were able to interpret Franklin's X-ray data to determine the double helix structure of DNA.

While other tools were used in the process of understanding DNA, X-ray diffraction was crucial in revealing the structure of the molecule itself.