Which substance can not be decomposed by ordinary chemical means?

(1) methane (3) ethanol

(2) mercury (4) ammonia

(2) mercury cannot be decomposed by ordinary methods because it is an element, to "decompose" these you need an accelerated atom smasher to separate these into protons, neutrons, electrons, etc.

Elements can't be decomposed while compounds can be decomposed.

The answer would be (2) mercury because since mercury is an element, it cannot be broken down chemically.