I'm having a hard time with Redox and how to figure out which is redox

Question

Select all of the reactions that are redox reactions.
I) Ca + 2H2O  Ca(OH)2 + H2
II) CaO + H2O  Ca(OH)2
III) Ca(OH)2 + H3PO4  Ca3(PO4)2 + H2O
IV) Cl2 + 2 KBr  Br2 + 2 KCl

I and II
II and III
I and IV
III and IV

What is the oxidizing agent in the following reaction?
Zn(s) + NO31-(aq)  Zn(OH)42-(aq) + NH3(aq)

Zn(s)
NO31-(aq)
Zn(OH)42-(aq)
NH3(aq)

And no this isnt homework just "practice probs" and no answer...i don't know how to do them or understand them thanx