Final answer:
The DNA double helix consists of two antiparallel strands where adenine pairs with thymine and cytosine pairs with guanine. The correct option is d.
Step-by-step explanation:
The correct statement about the structure of a DNA double helix is that (d) the two DNA strands run antiparallel. This means that the 3' end of one strand faces the 5' end of the other strand.
Base pairing within the helix occurs between a purine and a pyrimidine, which means adenine (A) pairs with thymine (T) and cytosine (C) pairs with guanine (G).
Adenine and thymine are connected by two hydrogen bonds, while cytosine and guanine are connected by three hydrogen bonds. This structure is stabilized by covalent bonds between the phosphate group of one nucleotide and the sugar of the next.
The statements that the two strands are identical, purines pair with purines, thymine pairs with cytosine, and the nucleotides are ribonucleotides, are incorrect regarding the DNA double helix.
Also, it's important to note that DNA nucleotides contain a deoxyribose sugar, not ribose, which is found in ribonucleotides (RNA). The correct option is d.