The correct option is A but-1-ene
The carbon chain has four carbon atoms in the parent chain. So, the word root is 'but'.
The chain should be numbered in such a way that the carbon atom containing the double bond gets the lowest number a shown below.
CH2=CH2−CH2−CH3 1 2 3 4
Hence the IUPAC name of the compound is 'but-1-ene'.