Nucleic acids differ in two systematic ways at the monomer level, and recalling both the sugar and the pyrimidine base resolves this comparison. RNA is built on the sugar ribose, which retains the hydroxyl group at the C2 position, whereas DNA is built on 2-deoxyribose, which lacks that C2 hydroxyl, the missing oxygen being the origin of the name deoxy. For the bases, RNA uses adenine, guanine, cytosine, and uracil, while DNA replaces uracil with thymine (which is 5-methyluracil); the other three bases are common to both. Therefore RNA is characterised by ribose plus uracil and DNA by deoxyribose plus thymine, making the first option correct. The second option swaps the two molecules entirely and is wrong. The third option pairs ribose with thymine, but thymine belongs to DNA, so it is incorrect. The fourth option is false because the sugars are not identical and the phosphate group is the same in both. This matches the NCERT comparison of DNA and RNA. As a consistency check, the absence of the C2 hydroxyl makes DNA more chemically stable than RNA, which fits its role as the long-term genetic store.