Which of the following process is endothermic?

Question

$\begin {array} {1 1} (A)\;C(graphite) \rightarrow C(diamond) & \quad (B)\;C(s)+O_2(g) \rightarrow CO_2(g) \\ (C)\;\text{Combustion of carbon monoxide} & \quad (D)\;Cl(g) \rightarrow Cl^-(g) \end {array}$