Well as mithoron said.its about symmetry.Let's see how but-1-ene comes out to be the answer
On addition of HBr markownikov rule is followed but lets draw both product.
1.)CH2=CH2 + HBr--->Br-CH2-CH3 + CH3-CH2-Br (both products are 1-bromoethane)
2)CH2=CH-CH2-CH3 + HBr--->Br-CH2-CH2-CH2-CH3 + CH3-BrCH-CH2-CH3 (notice the name of both the product. 1st one is 1-bromobutane. 2nd one is 2-bromobutane)
3)CH3-CH=CH-CH3 + HBr---->CH3-BrCH-CH2-CH3 + CH3-CH2-BrCH-CH3 (both products are same i.e both are 2-bromobutane)
4)CH3-CH2-CH=CH-CH2-CH3 + HBr--->CH3-CH2-BrCH-CH2-CH2-CH3 +
CH3-CH2-CH2-BrCH-CH2-CH3 (here also both the compounds are same i.e both are 3-bromohexane)
So what distinguishes the 2nd one from these three is that the second molecule is unsymmetrical.You can find its unsymmetrical by looking at the no:of carbon to both side of double bond.If they are equal then molecule is symmetrical.