Which pair of oxides is acidic in nature?

We need to identify the pair of oxides that are both acidic in nature.

$$B_2O_3$$ (boron trioxide) is a well-known acidic oxide. It reacts with water to form boric acid and with bases to form borates.

$$SiO_2$$ (silicon dioxide) is also an acidic oxide. It reacts with strong bases to form silicates, though it does not dissolve in water readily.

CaO (calcium oxide) and BaO (barium oxide) are basic oxides, as they are oxides of Group 2 metals. $$N_2O$$ (nitrous oxide) is a neutral oxide.

Therefore, the pair of oxides that are both acidic is $$B_2O_3$$ and $$SiO_2$$.

The correct answer is Option (4).