Which of the following is an unsaturated hydrocarbon?

Hydrocarbons are classified into two types based on the type of bonds they contain: saturated hydrocarbons and unsaturated hydrocarbons. In saturated hydrocarbons; there is only one bond between two carbon atoms, which are known as alkanes. 
Unsaturated hydrocarbons are further classified into two types based on the bond formed between two carbon atoms. That is to say, double and triple-bonded carbon. Alkene is a double-bonded carbon, and alkyne is a triple-bonded carbon.
In the question, ethene is an unsaturated hydrocarbon because the rest are alkane examples.