Final answer:
Rosalind Franklin used X-ray diffraction to demonstrate the helical nature of DNA, while Watson and Crick formulated the structural model of DNA.
Step-by-step explanation:
Rosalind Franklin used X-ray diffraction methods to demonstrate the helical nature of DNA, while Watson and Crick formulated the double stranded structural model of DNA. Franklin's X-ray diffraction images of DNA provided the crucial information that allowed Watson and Crick to confirm that DNA forms a double helix and determine details of its size and structure.