Redox reactions are those in which the oxidation numbers of the elements involved are changed.
Equation 1:
2Na(s) + Cl2(g) --> 2NaCl
The oxidation numbers of Na and Cl in the reactant side are both zero because they are in elemental form. In the product side, however, the oxidation numbers are +1 and -1, respectively. Hence, this is an example of redox reaction.
Equation 2:
Cd(s) + Pb+2(aq) --> Cd2+(aq) + Pb(s)
The oxidation numbers of Cd and Pb+2 in the reactant side are 0 and +2, respectively. They are, however, +2 and 0 in the product side. Hence, this is also a redox reaction.
Equation 3:
Pb(NO3)2(liq) + 2LiCl(aq) --> PbCl2(s) + 2LiNO3(aq)
The oxidation numbers of the involved ions (both cations and anions) are not changed. Hence, this is NOT an oxidation reaction.
Equation 4:
C(s) + O2(g) --> CO2(g)
Just as the equation 1 and 2, the oxidation numbers of the reactants are not similar to those in the product. Hence, this is an example of oxidation reaction.