The DNA molecule has uniform diameter due to

DNA molecule has uniform diameter due to specific base pairing between purine and pyrimidine. In DNA, the bases adenine and guanine form hydrogen bonds with their complementary pyrimidines; thymine and cytosine, respectively, this is called as complementary base pairing.
Most DNA molecules consist of two biopolymer strands coiled around each other to form a double helix. The two DNA strands are known as polynucleotides since they are composed of simpler units called nucleotides. Each nucleotide is composed of a nitrogen-containing nucleobase either guanine (G), adenine (A), thymine (T), or cytosine (C) as well as a monosaccharide sugar called as deoxyribose and a phosphate group.
So, the correct answer is option C.