Final answer:
The reagent that can selectively oxidize an aldose to an aldonic acid is Ag₂O in NH₄OH, known as Tollens' reagent, option B.
Step-by-step explanation:
Among the given options, the reagent that will selectively oxidize an aldose to an aldonic acid is B. Ag₂O in NH₄OH, also known as Tollens' reagent. Tollens' reagent is known for its ability to oxidize aldehydes to carboxylic acids while leaving ketones unaffected. The aldose sugar, in an alkaline medium, reduces the Ag+ ion from the Tollens' reagent to metallic silver and is itself oxidized to an aldonic acid. For instance, D-glucose can be oxidized to D-gluconic acid by this reagent.
Other options like Uv Light (A), FeCl₃ (C), and HNO₃ (D) are not known for their selective oxidation of aldoses to aldonic acids. UV light does not specifically oxidize aldehydes, FeCl₃ is typically not used as an oxidizing agent in this context, and HNO₃ is a strong oxidizing agent that may lead to overoxidation beyond the aldonic acid stage.