Which oxide of $Mn$ is acidic in nature?

Which of the following mixtures forms chromic acid?

In the structure of the dichromate ion,there is a :

Each of the following ions contains vanadium in the $+V$ oxidation state except:

$MnO_2$ when fused with $KOH$ and oxidised in air gives a dark green compound $X$. In acidic solution,$X$ undergoes disproportionation to give an intense purple compound $Y$ and $MnO_2$. The compounds $X$ and $Y$,respectively,are

The amphoteric oxide among $V_2O_3$,$V_2O_4$,and $V_2O_5$ upon reaction with alkali leads to the formation of an oxide anion. The oxidation state of $V$ in the oxide anion is: