Decomposing chemical compounds - Ease of Decomposition
It really depends on what the definition of "decompose" is in this case. If you take the definition of "decompose" to be:


  ... to break down or cause to break down into component elements or simpler constituents


then the only option would be ammonia because the others are elements whereas ammonia is a compound which can be broken down into elemental Hydrogen and Nitrogen by the Haber Process. The energetics is not of concern in this case.