An olefin (or alkene) is a type of hydrocarbon that contains at least one carbon-carbon double bond. The general formula for alkenes is \( C_nH_{2n} \).
Now let's assess the options:
A) \( C_2H_5OH \) - This is ethanol, which is an alcohol, not an alkene.
B) \( C_2H_6 \) - This is ethane, which is an alkane (saturated hydrocarbon), not an alkene.
C) \( C_2H_4 \) - This follows the formula \( C_nH_{2n} \) where \( n=2 \). This is ethene (or ethylene), which is an alkene.
D) \( C_2H_4O \) - This formula has an oxygen atom as well, which typically indicates a functional group that could be associated with an alcohol, ether, or ketone, depending on the structure. It isn't an alkene.
Thus, the correct answer is:
C) C2H4