Final answer:
Watson and Crick concluded that DNA is a double helix using X-ray crystallography, particularly utilizing the detailed X-ray diffraction patterns provided by Rosalind Franklin's pioneering work.
Step-by-step explanation:
Watson and Crick based their conclusion that DNA is a double helix mainly on experimental results and measurements from X-ray crystallography. This technique was used by Rosalind Franklin to demonstrate the helical nature of DNA, whose data Watson and Crick utilized to formulate the double stranded structural model of DNA. Important details like the 0.34 nm repeat dimension and the presence of the major and minor grooves in the DNA helix were discerned through the X-ray diffraction patterns produced by Franklin's work. These patterns were pivotal in aiding Watson and Crick to propose that the structure of DNA consists of two side-by-side polynucleotide chains wrapped into the shape of a double helix. Sadly, Rosalind Franklin's critical contributions were recognized only posthumously, as Nobel Prizes are not awarded posthumously, and she passed away before the Nobel Prize was awarded to Watson, Crick, and Wilkins in 1962 for their work on the DNA structure.