(a) Substance X is likely to be lead nitrate, because on reacting with potassium iodide it forms a yellow precipitate.
(b) The yellow solid or precipitate consists of lead iodide, which is one of the products in this reaction.
(c) This chemical reaction is characterised by the formation of precipitate.
(d) 2KI (s) + Pb(NO3)2 (aq) PbI2 (s) + 2KNO3 (aq)