Answer:
B. Cu + 4HNO3 → Cu(NO3)2 + 2H2O
Step-by-step explanation:
A redox reaction is one in which oxidation and reduction reactions are occuring simultaneously.
In a redox reaction, an oxidizing agent is reduced ( its oxidation state decreases) whereas a reducing agent is oxidized (it's oxidation state increases).
The nitrogen in the nitrate (v) ion, NO3-, exists in the +5 oxidation state. Therefore it would be acting as an oxidizing agent in any reaction in which it's oxidation state becomes less than +5 after the reaction.
Let us then consider each of the reactions:
A. Reaction 1: in this reaction, the oxidation state of nitrogen remains +5 as what is simply an exchange of radicals in a neutralization reaction.
B. Reaction 2: in this reaction, trioxocarbonate (v) acid oxidizes Copper atom to Copper (ii) ion, while itself it is reduced to Nitrogen (IV) oxide. The oxidation state of nitrogen changes from +5 to +4
C. Reaction 3: in this reaction, what occurs is an exchange of radicals by a displacement reaction. The oxidation state of nitrogen remains the same
D. Reaction 4: in the reaction, what occurs is a neutralization reaction, and the oxidation state of nitrogen remains the same.
Therefore, the correct option is B.