Which of the following pairs of oxides is amphoteric in nature?

The incorrect statement$(s)$ among $(a)-(c)$ is (are) :-
$(a)$ $W(VI)$ is more stable than $Cr(VI)$
$(b)$ In the presence of $HCl$,permanganate titrations provide satisfactory results.
$(c)$ Some lanthanoid oxides can be used as phosphors.

Iron is rendered passive by the action of

Explain the trends in $M^{2+}/M$ standard electrode potentials.

Select the incorrect statement.

The most acidic compound in water is