The answer is simply butene. You can also write it as But-1-ene. Since the double bond is between the first and second carbon the '1' is unnecessary. The answer butene would have been wrong if the double bond was between the second and the third carbon.
The I.U.P.A.c. name of the molecular formula CH3CH2CH=CH2 is But-1-ene. I can explain my answer.
This molecular formula has four carbons and one double bond at C1. According to the naming conventions of hydrocarbons, four carbon atoms are known as “But” and the double bond is called “Ene”. Hence, its IUPAC name will be But-1-ene.
Butene is not the correct answer as it has not specified the position of the double bond.