which of the following substance in the gaseous state should be called a vapour? and why

(a) heliun

(b) nitrogen

(c) carbon dioxide

(d) sulphur

1) Helium, Nitrogen and carbon dioxide exist as gases at room temperature. That is their natural state of matter is gaseous phase. Sulphur is the only substance which exist as a solid at room temperature and on heating changes into gaseous phase. Therefore sulphur in gaseous state should be called vapour. 

2) The term vapour is used to denote the gaseous phase of a substance which exists as a solid or a liquid at room temperature (i.e. its natural state of matter is a solid or a liquid). In other words, vapour is formed when a substance undergoes a change of state from solid or liquid to gaseous state. 

Thus as sulphur is the only one out of the four that has undergone a phase change, the term vapour should be used for sulphur. The remaining three already exist as gases in their natural state.