Which of the following statements about DNA forms are true?

• A. I, II, III, IV
• B. I, II
• C. III
• D. IV

Answer: (D)

DNA can adopt more than one helical form. Under normal cellular conditions, B-DNA is the standard right-handed form. When conditions favor dehydration or when dealing with RNA-DNA hybrids, the molecule can adopt A-DNA, which is also right-handed but wider and shorter. Z-DNA is a distinct left-handed form that can arise in sequences rich in CG repeats under high salt or torsional stress; it has a zigzag backbone and is generally less stable than B-DNA, often transient and associated with regulatory roles in transcription.

If the fourth statement asserts that DNA can exist in multiple recognized forms (for example, mentioning B, A, and Z forms), it aligns with established observations about DNA structure. The other statements, if they claim incorrect handedness, impossible conditions, or that only a single form exists, would not fit the true picture of DNA forms. To explain precisely why the fourth statement is the right one for this item, I’d need the exact wording of statements I–IV.