(i) (a) Zinc is more reactive than silver. It will displace silver from silver nitrate solution.
(b) Magnesium is more reactive than iron. It will displace iron from iron (II) chloride solution.
(c) Copper is less reactive than magnesium. It will not displace magnesium from magnesium sulphate solution.
(ii) X is potassium (K).
Potassium being soft can be cut by a knife and being lighter than water hence, it will float at the surface. The heat produced during reaction with water will melt it.