Final answer:
A pentose sugar has five carbons, which is represented by option D. 5. Ribose and deoxyribose are both examples of pentose sugars.
Step-by-step explanation:
A pentose sugar, which means the sugar has a five-carbon backbone. In the context of nucleic acids, two significant 5-Carbon carbohydrates are mentioned: ribose, which is present in RNA, and deoxyribose, found in DNA. Ribose and deoxyribose are both pentose sugars and, as such, they each have five carbons. These carbons are denoted as 1', 2', 3', 4', and 5'. The primary difference between these two sugars is on the 2' carbon - ribose has a hydroxyl (-OH) group attached, while deoxyribose has only a hydrogen atom due to the absence of an oxygen atom, thus giving the name deoxyribose. Accordingly, the answer to the student's question is D. 5.