which one is redox reaction

Question

Please, let me know how to detect which of the following equations is a redox reaction?

1. Mg + Cl2 MgCl2 

2. CaO + H2O Ca(OH)2 

3. HNO3 + NaOH NaNO3 + H2O 

4. NaCl + AgNO3 AgCl + NaN

Answer 1

Mg(s) + Cl2 (g) → MgCl2 (s).