Final answer:
The true statement about a DNA double helix is that the backbone is composed of sugar-phosphate-sugar-phosphate chains.
Step-by-step explanation:
The correct statement regarding a DNA double helix is D) The backbone is composed of sugar-phosphate-sugar-phosphate. In the DNA double helix, the two strands are anti-parallel, not parallel as stated in option A. This means they run in opposite directions relative to each other.
Adenine does not pair with guanine; it pairs with thymine through two hydrogen bonds, while guanine pairs with cytosine through three hydrogen bonds, which contradicts option B. Lastly, the structure of DNA is a double helix, not a single-stranded helix, so option C is also incorrect.