The secondary carbocation shown rearranges by a single 1,2-shift to the most stabilised cation reachable by one shift. Which structure shown is that cation?

The cation shown is the 2-methylcyclohexyl cation. A 1,2-hydride shift from the adjacent ring carbon bearing the methyl group moves the charge to that carbon. This gives the 1-methylcyclohexyl cation, which is tertiary and therefore more stable than the secondary cation; the other structures are not formed by a single stabilising shift.