The question is incomplete, here is a complete question.
Which formula represents an alkene?
(a) ![CH_4](https://tex.z-dn.net/?f=CH_4)
(b) ![C_2H_4](https://tex.z-dn.net/?f=C_2H_4)
(c) ![C_2H_6](https://tex.z-dn.net/?f=C_2H_6)
(d) ![C_2H_5OH](https://tex.z-dn.net/?f=C_2H_5OH)
Answer : The correct option is, (b) ![C_2H_4](https://tex.z-dn.net/?f=C_2H_4)
Explanation :
Hydrocarbons are the type of organic compound that contains only hydrogen and the carbon atoms.
Saturated hydrocarbon : These are those hydrocarbons in which all carbon atoms and hydrogen atoms are bonded together with single bonds. The general formula of saturated hydrocarbon (alkane) is, ![C_nH_{2n+2}](https://tex.z-dn.net/?f=C_nH_%7B2n%2B2%7D)
For example : Methane, Ethane, etc.
Unsaturated hydrocarbon : These are those hydrocarbons in which carbon-carbon atom bonded together with double or triple covalent bonds.
The general formula of unsaturated double bond hydrocarbon (alkene) is, ![C_nH_{2n}](https://tex.z-dn.net/?f=C_nH_%7B2n%7D)
The general formula of unsaturated triple bond hydrocarbon (alkyne) is, ![C_nH_{2n-2}](https://tex.z-dn.net/?f=C_nH_%7B2n-2%7D)
For example : Ethene, Butyne, etc.
As per question,
(a)
: It is a saturated hydrocarbon and the name of this hydrocarbon is methane.
(b)
: It is an unsaturated hydrocarbon and the name of this hydrocarbon is ethene.
(c)
: It is a saturated hydrocarbon and the name of this hydrocarbon is ethane.
(d)
: It is a saturated hydrocarbon and the name of this hydrocarbon is ethanol.
Hence, the formula represents an alkene is, ![C_2H_4](https://tex.z-dn.net/?f=C_2H_4)