The stability of isomeric alkenes increases with the number of alkyl groups attached to the doubly bonded carbons, because alkyl groups donate electron density and provide hyperconjugative stabilization through overlap of C-H sigma bonds with the pi system. Counting alkyl substituents on the double bond: but-1-ene is monosubstituted, both but-2-enes are disubstituted, and 2-methylpropene, $(C{H}_3{)}_{2}C=C{H}_2$, is also disubstituted but its substitution is on a single carbon giving the greatest number of hyperconjugative structures, making it the most stable here. 2-Methylpropene has six alpha C-H bonds available for hyperconjugation, more than the four available to but-2-ene, so it is the most stable. But-1-ene is least stable because it is only monosubstituted, so it is wrong. cis-But-2-ene is destabilized relative to the trans form by steric strain between methyl groups, so it is not the answer. trans-But-2-ene is more stable than the cis isomer but still has fewer hyperconjugative interactions than 2-methylpropene, so it is incorrect. This reflects the NCERT and JEE treatment of alkene stability. A sanity check: greater alkyl substitution and more alpha hydrogens reliably predict higher stability, favouring 2-methylpropene.