$\begin{array}{1 1} \text{(a) +4 to +1 , so the carbon is reduced } \\ \text{(b) +2.5 to +4 , so the carbon is reduced } \\ \text{(c)+2.5 to +4 , so the carbon is oxidized } \\ \text{(d)-2.5 to -4 , so the carbon is oxidized } \end{array} $