The pentose sugar in DNA and RNA has A. Open chain structureB. Pyranose structureC. Furanose structureD. All of the above

D. All of the above.

The pentose sugar in both DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) is called ribose. Ribose can exist in all three forms mentioned:

A. Open chain structure: Ribose can exist in an open chain form where it is not cyclic and lacks the ring structure.

B. Pyranose structure: Ribose can form a pyranose ring structure, specifically a furanose ring structure, where the five-carbon sugar forms a ring with four carbon atoms and one oxygen atom. The ring structure is reminiscent of a pyran ring.

C. Furanose structure: Ribose can also form a furanose ring structure, which is a five-membered ring with four carbon atoms and one oxygen atom. This is the more common form found in RNA.

Therefore, the correct answer is D. All of the above.