Watson and Crick's double-helical model was proposed

Solution

Watson and Crick's double-helical model of DNA was proposed based on both: (A)   Chargaff's chemical analysis of DNA, which showed that the amount of adenine (A) is equal to the amount of thymine (T) and the amount of guanine (G) is equal to the amount of cytosine (C) in DNA. This observation led to the idea of complementary base pairing in the DNA molecule. (B)   Wilkins and Franklin's X-ray diffraction data, which provided insights into the structure of DNA, particularly its helical nature and dimensions. This data helped confirm and refine the double-helical model proposed by Watson and Crick.