Explanation

(b)
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It is chromium peroxide.
Let the oxidation number of Cr is $''x^{''}.$
${Cr}^{x +} + O_{2}^{-} + O^{2 -} + O_{2}^{-} - {CrO}_{5}$
$x + ( - 1)2 + ( - 1)2 + ( - 2)1 = 0$
$x - 6 = 0$
$x = + 6$
Hence, the oxidation state of Cr is +6.