a) Double replacement precipitation reaction.
b) Lead nitrate and potassium iodide.
c) This reaction will not remove lead ions from waste water.
For a complete answer to the a) of your question Socratic.org is the best place to look.
b) Nitrogen and oxygen combine to form nitrates and their salts occur naturally. Lead nitrate was once used in the pigmentation of paints, which proved to be lethal to painters. Potassium and iodine combine to form the soluble salt potassium iodide used as a medicine.
c) It is an interesting idea, to have a couple of soluble salts react with lead ions in waste water to remove the lead ions. But this equation is already balanced. That means dumping appropriate amounts of the two salts into lead contaminated water will have no effect in reducing the pollution.
Any lead ions picked up from the water during the reaction will be replaced by lead ions from the