The precipitation means the solid form of the compound. Among the given compounds (a) Calcium nitrate Ca(NO₃)₂ is highly soluble and remains in ionic form in the solvent and will not make precipitation. (b) Copper (I) nitrate [Cu(NO₃)] is unstable as copper (II) is always more stable than its mono ionic form thus it can not precipitated in a reaction. (c) Iron (I) chloride (FeCl) is also unstable as Fe only stabilizes in its +3 or +2 form. (d) Iron (II) sulfide (FeS) is precipitated most likely and is an renowned ore of iron. (e) Potassium sulfate (K₂SO₄) is like to remain mostly in its ionic form and has less tend to precipitate.
Thus FeS will be more likely to be precipitated.