Which of the following reactions are redox reactions? #A. Ba+Mg^(2+)rarrBa^(2+) + Mg.# #B. H^+ +HO^(-)rarrH_2O# #C. CO_2(g) +H_2O(l)rarrHCO_3^(-) + HO^(-)# #D. Cr(s) + 3H^(+)rarrCr^(3+) + 3/2H_2#
In general, metals are electron rich species. Given that oxidation is loss of electrons, metals tend to be oxidized. On the other hand, over on the other side of the Periodic Table, non-metals tend to have greater nuclear charges, and tend to gain electrons; they are thus oxidizing, and tend to be reduced, cf combustion.
You probably should learn about oxidation numbers.