The reaction is decomposition but it’s not redox reaction 

• Among the given decomposition reactions, in case of                   ${\mathrm{MgCO}}_3\to \mathrm{MgO}+{\mathrm{CO}}_2$ 
• Magnesium carbonate is decomposed to magnesium oxide and carbon dioxide. 

           ${\mathrm{Mg}}^{2+}+{\mathrm{CO}}_3^{2-}\to {\mathrm{Mg}}^{2+}+{\mathrm{O}}^{2-}+{\mathrm{CO}}_2$

The charge on both sides remain the same i.e. there is no change in the charge here unlike in other reactions. Hence, ${\mathrm{MgCO}}_3\to \mathrm{MgO}+{\mathrm{CO}}_2$ is a decomposition reaction but not a redox reaction.