The correct answer is option a. "Double-stranded regions of RNA typically take on an B-form right-handed helix".
Explanation:
Most of the native double-stranded DNA is on an B-form right-handed helix, following the structure proposed by Watson and Crick with about 10–10.5 base pairs per turn. However, double-stranded RNA does not follow this structure, and most regions have an A-form structure. The A-form right-handed helix have slightly more base pairs per turn, which makes it 20-25% shorter than B-DNA.
