Which element forms the maximum number of stable oxides?

The oxoanion which contains all equivalent $M-O$ bonds is:
$(I)$ $CrO_4^{2-}$
$(II)$ $MnO_4^-$
$(III)$ $Cr_2O_7^{2-}$

Cigarette or gas lighter is made up of

Which of the following oxides is amphoteric?

$A$ purple colored compound of Manganese $(X)$ decomposes on heating to liberate oxygen and forms compounds of Manganese $(Y)$ and $(Z)$. Compound $(Z)$ reacts with $KOH$ in the presence of potassium nitrate to give compound $(Y)$. Compounds $(X)$,$(Y)$,and $(Z)$ respectively are:

Which of the following oxidation states of chromium forms the most stable compounds?