b
\(\mathop {Cr}\limits^X {\mathop O\limits^{ - 2} _2}{\mathop {Cl}\limits^{ - 1} _2}\) : \(X + 2\left( { - 2} \right) + 2\left( { - 1} \right) = 0\)
\(\therefore X = + 6\)
\(MnO_4^ - \,\,:\) \(X + 4\left( { - 2} \right) = - 1\)
\(\therefore X = + 7\)