What defines an alkene?

An alkene is specifically defined as a type of hydrocarbon that contains at least one double bond between carbon atoms. The presence of this double carbon-carbon bond distinguishes alkenes from other types of hydrocarbons. Alkenes follow the general formula CnH2n, indicating that they have fewer hydrogen atoms than alkanes, which only contain single bonds.

The other answer choices describe different types of molecules or classifications that do not correctly identify alkenes. Hydrocarbons with a single bond refer to alkanes, which are saturated and contain only single bonds. Hydrocarbons with a triple bond refer to alkynes, another category of unsaturated hydrocarbons. Lastly, stating that hydrocarbons have only carbon atoms is too vague, as it doesn't specify the type of bonds present and could apply to various classes of hydrocarbons, including both alkanes and alkenes. Thus, the defining feature of alkenes is the presence of the double bond, making the correct answer the one that specifies this characteristic.