$(a)$ Carbon cannot be used as a reducing agent to obtain $Mg$ from $MgO$. Why?
$(b)$ How is sodium obtained from molten sodium chloride? Give the equations of the reactions.
$(c)$ How is copper obtained from its sulphide ore? Give the equations of the reactions.

(N/A) $Mg$ has a higher affinity for oxygen than carbon does,meaning $MgO$ is more stable than $CO_2$ at the temperatures required for reduction.
$(b)$ Sodium is obtained by the electrolytic reduction of molten sodium chloride $(NaCl)$.
At cathode: $2Na^+ + 2e^- \rightarrow 2Na$
At anode: $2Cl^- \rightarrow Cl_2 + 2e^-$
$(c)$ Copper is obtained from its sulphide ore $(Cu_2S)$ by roasting followed by self-reduction:
$2Cu_2S + 3O_2 \rightarrow 2Cu_2O + 2SO_2$
$2Cu_2O + Cu_2S \rightarrow 6Cu + SO_2$