Based on structure, in what way do dna molecules differ from rna molecules?

Unlike DNA, RNA is almost always a single-stranded molecule and has a much shorter chain of nucleotides. RNA contains ribose, rather than the deoxyribose found in DNA (there is a hydroxyl group attached to the pentose ring in the 2' position whereas RNA has two hydroxyl groups). These hydroxyl groups make RNA less stable than DNA because it is more prone to hydrolysis. Several types of RNA (tRNA, rRNA) contain a great deal of secondary structure, which help promote stability.

Explanation:

What is a RNA molecule?

Well, RNA consist of aminoacids. An aminoacid is built up of three bases, which is a micture of a nucleoside (organic molecule), phosphatgroup and the base (A,T,G,C). This package is also called nucleic acid.

So RNA is built up the same way as DNA, but it's just a helix and not a duople-helix (DNA) and has got the base U instead of the base T. You can see that RNA is a molecule because its built up of molecules.

i think the answer is b.