Which of the following metal-oxide is acidic as well as basic?

The metallic oxides that can react with acids and bases are known as amphoteric oxides. These compounds behave as bases when they react with acids and form normal salts.

These salts also behave as acids when they react with specific bases and form a complex salt as they cannot replace the metallic ion of the base.

${\mathrm{Al}}_2{\mathrm{O}}_3+6\mathrm{HCl}\to 2{\mathrm{AlCl}}_3+3{\mathrm{H}}_2\mathrm{O}$ (Reaction of Al₂O₃ with acid)

${\mathrm{Al}}_2{\mathrm{O}}_3+2\mathrm{NaOH}+3{\mathrm{H}}_2\mathrm{O}\to 2\mathrm{NaAl}{\left(\mathrm{OH}\right)}_4$ (Reaction of Al₂O₃ with base).

Here NaAl(OH)₄ is a complex salt because aluminium being less reactive than sodium, cannot replace sodium from the base. Hence, it  combines with sodium to form a complex salt.