(a)
A species is amphoteric if it is soluble in acid (behaves as a base) as well as in base (behaves as an acid).
$SnO_{2}$ is an amphoteric oxide.
$\ \ \ \ SnO_{2} + 4HCl \longrightarrow SnCl_{4} + 2H_{2}O$
$SnO_{2} + 2NaOH \longrightarrow Na_{2}SnO_{3} + H_{2}O$