$(ii)$ $Cr _{2} O _{3}$ is an amphoteric oxide.

$(iii)$ $\operatorname{In} RuO _{4},$ $'Ru'$ is in $+8$ oxidation state. So it act as oxidising agent.

$(iv)$ Red colour of ruby is due to the presence of $Cr ^{+3}$ ions in $Al _{2} O _{3}$