Which of the following is most likely to undergo hydride shift?
Besides Avinsh's comment, there is another factor.  When you have a carbocation center next to a cyclopropane ring or cyclobutane ring, the "bent bonds" of the ring partially overlap the "vacant" orbital on the cation center.  To the extent that this occurs, the formally primary ion (c) is something like a more stable tertiary one.  Ion (a) does not have this factor because the cation center is already inside the ring instead of next to it.