The primary carbocation shown rearranges by a single 1,2-shift. Which structure shown is the most stable cation obtainable in this way?

The cation shown is the cyclopentylmethyl cation. A 1,2-hydride shift from the ring CH gives the 1-methylcyclopentyl cation, which is tertiary; ring expansion would give only the secondary cyclohexyl cation. The tertiary cation is more stable than the secondary one, so the 1-methylcyclopentyl cation is the most stable single-shift product.