Which of the following oxides is acidic in nature?

1. ${\mathrm{B}}_{2}{\mathrm{O}}_{3}$

2. ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$

3. ${\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$

4. ${\mathrm{In}}_{2}{\mathrm{O}}_{3}$

Hint: Down the group basic character increases.

The acidic nature of oxides changes from acidic to basic through amphoteric on moving down the group.