The secondary carbocation shown first undergoes ring expansion and the resulting cation then undergoes one 1,2-hydride shift. Which structure shown is the most stable cation at the end of this sequence?

The cation shown is the 1-cyclobutylethyl cation. Ring expansion (a ring bond migrates to the cationic carbon) gives the secondary 2-methylcyclopentyl cation and relieves ring strain. A 1,2-hydride shift from the neighbouring ring carbon that bears the methyl group then gives the tertiary 1-methylcyclopentyl cation, the most stable cation of the sequence.