The primary carbocation shown undergoes ring expansion by a 1,2-alkyl shift (a ring C-C bond migrates to the cationic carbon). Which structure shown is the cation formed?

The cation shown is the cyclobutylmethyl cation. Migration of a ring C-C bond to the CH${}_2^{+}$ carbon enlarges the four-membered ring to a five-membered ring and leaves the charge on a ring carbon. The product is the cyclopentyl cation, which is secondary rather than primary and also relieves the ring strain of cyclobutane.