In 1953, the scientists James Watson and Francis Crick published their landmark findings on the structure of DNA. Watson and Crick deduced the structure of DNA by unifying evidence that they collected from several scientists who were also seeking to answer this important question. Identify the pieces of evidence describing the features of DNA that Watson and Crick used to determine the structure of DNA.

A purine base forms covalent bonds with a pyrimidine base located on the opposite DNA helix.
DNA forms a right-handed double helical structure with two polynucleotide chains coiled around a central axis.
The two chains are parallel, both running in a 5' to 3' direction.
A purine base forms hydrogen bonds to pair with a pyrimidine base located on the opposite DNA strand. Specifically, A pairs with T, and C pairs with G.
The sugar-phosphate backbones of each DNA helix run antiparallel to one another.
The diameter of the DNA double helix is 2 nanometers, with each purine-pyrimidine base pair spanning an equivalent distance between the two chains.

The correct answers are statements 2nd, 4th, 5th and 6th.

Explanation:

The X-ray experiments performed by Rosalind Franklin and the experiments performed by Watson and Crick resulted in the discoveries of the DNA model. In the 1950s, the X-ray diffraction technique was used by Franklin to know more about the composition of DNA. With the assistance of the experiment done by Franklin, Watson and Crick deduced the model of the DNA helix.  

The prime contribution in finding the molecular composition of DNA was done by Watson and Crick and was helped by the contributions done by Chargaff, Hershey and Chase, Linus Pauling, Franklin and Wilkins, and others.  

Based on the X-ray experiment conducted by Franklin, it came to know that DNA produces a right-handed helical composition with two polynucleotide chains enveloped around a central axis.  

Based on the discoveries made by Chargaff, a purine base comprises hydrogen bonds, which pairs with a pyrimidine base situated on the DNA strand present on the contrary. It came to know that A forms pairing with T and C forms pairing with G.  

Alexander Todd an organic chemist discovered that the sugar-phosphate backbones of every DNA helix move antiparallel to each other.  

Based on the X-ray experiment performed by Franklin, it came to know that 2 nm is the diameter of the DNA double helix, and each purine-pyrimidine base-pairing covers an equal distance between the two chains.  

i am positive it is c. condensing but i may be wrong