In chemistry, a precipitation reaction is a double replacement reaction. It involves two reactants and two products. It means that the cations and anions of the compounds interchange with one another to yield two products. But what makes precipitation reactions different is that when two liquid reactants are allowed to react together, a solid substance called precipitate is formed. The solid appears because it is insoluble to the other product in aqueous state.
Therefore, basing on the choices given, precipitation reactions apply to letters C and D.