What technique did Wilkins and Franklin use to determine the shape of DNA?

Wilkins and Franklin used X-ray diffraction as their primary technique to investigate the structure of DNA. This method involves directing X-rays at a crystalline sample of DNA and analyzing the resulting diffraction pattern. The arrangement and intensity of the scattered X-rays produce a characteristic pattern that provides critical information about the molecular structure of the sample.

Franklin's famous Photograph 51, obtained through X-ray diffraction, revealed key details, including the helical shape of the DNA molecule. Wilkins played a crucial role in this process by collaborating with Franklin and utilizing her findings to contribute to understanding DNA's structure, which ultimately led to the discovery of the double helix.

The other methods listed, such as mass spectrometry, ultracentrifugation, and electron microscopy, do not provide the same type of structural detail about the DNA helix as X-ray diffraction does. Mass spectrometry is primarily used for determining the mass-to-charge ratio of ions, ultracentrifugation separates molecules based on their size and density but does not visualize structure directly, and electron microscopy, while useful for high-resolution imaging, was not the technique used by Wilkins and Franklin to elucidate the DNA structure specifically.