Covalence of carbon in the functional group of C and D are

$$\begin{gathered} {\mathrm{C}}_2{\mathrm{H}}_4\stackrel{\mathrm{HCl}/{\mathrm{AlCl}}_3}{\to }{\mathrm{CH}}_3-{\mathrm{CH}}_2^{+}\stackrel{\mathrm{KCN}}{\to }\underset{\mathrm{C}}{{\mathrm{CH}}_3-{\mathrm{CH}}_2-\mathrm{C}\equiv \mathrm{N}} \\ {\mathrm{CH}}_3-{\mathrm{CH}}_2^{+}\stackrel{\mathrm{AgCN}}{\to }\underset{\mathrm{D}}{{\mathrm{CH}}_3-{\mathrm{CH}}_2-\mathrm{N}\equiv \mathrm{C}} \end{gathered}$$

• Covalency of an element is defined as the number of covalent bonds formed by the atom of the element with other atoms.
• Hence, the covalency of carbon in the functional group of C and D are 4 and 3 respectively as it forms 4 and 3 covalent bonds respectively with the other atoms.