Independently, both Watson and Crick and their competitor Linus Pauling constructed an incorrect triple - helix model with the nitrogenous bases arranged so they were on the exterior of the molecule and the phosphate groups on the interior. What evidence caused Watson and Crick to revise their model?

Photo 51 produced by Rosalind Franklin

Explanation:

Watson and Crick inspired by the work of Linus Pauling on the structure of DNA tried to solve the mystery of the structure of the DNA.

In their one of the first attempts, they produced a model with three phosphate groups with magnesium in the interior portion and the nitrogenous bases on the exterior side.

This was not supported by the X-ray picture of the DNA called photo 51A which suggested that there is no magnesium present, in aqueous solution the phosphate is present on the exterior part with sugar and the nitrogenous bases are present in the interior portion.

Thus, Photo 51 A is the correct answer.