Final answer:
Bromine is the atom that is oxidized in the unbalanced reaction HNO₃ + HBr → NO + Br₂ + H₂O, as it goes from an oxidation state of -1 in HBr to 0 in Br₂.
Step-by-step explanation:
In the unbalanced reaction given, HNO₃ + HBr → NO + Br₂ + H₂O, we need to determine which atom is oxidized. To do this, we'll examine the change in oxidation states for the various atoms involved in the reaction.
Nitrogen changes from an oxidation state of +5 in nitric acid (HNO₃) to +2 in nitrogen monoxide (NO). This decrease in oxidation number indicates that nitrogen is being reduced. Bromine, on the other hand, changes from an oxidation state of -1 in hydrobromic acid (HBr) to 0 in molecular bromine (Br₂). Since the oxidation number of bromine increases, it is the element that is being oxidized. Therefore, the correct answer is d. bromine.