Which of the following is the correct order of acidic character?

Which of the following is not amphoteric?

When an excess of $KI$ is added to a $CuSO_4$ solution and then $Na_2S_2O_3$ is added to the mixture,which of the following statements is incorrect?

Highest oxidation state of manganese in fluoride is $+4$ $(MnF_4)$ but highest oxidation state in oxides is $+7$ $(Mn_2O_7)$ because $..........$

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.

Consider the following transformation $2 \ CuX_2 \xrightarrow[\text{Room}]{\text{temp.}} Cu_2X_2 + X_2 \uparrow$. Then $X^{-}$ can be: