(a) Metal M is sodium (Na), which is used as a coolant in nuclear reactors.
(b) Solution S is sodium hydroxide solution (NaOH). It is alkaline, it turns red litmus to blue.
(c) Gas G is hydrogen gas, which is lighter than air.
(d) 2Na (s) + 2H2O (l) 2NaOH (aq) + H2 (g) + HEAT
(e) This reaction is exothermic because it releases excessive heat.