Which one of these is not known?

Correct order of oxidising nature is

All $Cu(II)$ halides are known except $CuI_2$,because ....

Passivity of iron is due to the formation of

Arrange the oxides $CrO$,$CrO_3$,and $Cr_2O_3$ in the decreasing order of acidic strength.

The set of correct statements is:
$(i)$ Manganese exhibits $+7$ oxidation state in its oxide.
$(ii)$ Ruthenium and Osmium exhibit $+8$ oxidation state in their oxides.
$(iii)$ $Sc$ shows $+4$ oxidation state which is oxidizing in nature.
$(iv)$ $Cr$ shows oxidizing nature in $+6$ oxidation state.