The radius of a Bohr orbit is given by $r_n \propto n^2/Z$. For the same orbit (n is constant), the radius is inversely proportional to the atomic number Z. For H, Z=1. For He+, Z=2. Therefore, the new radius will be r/2. The question states He²⁺ which has no electrons and is thus incorrect; it should be He⁺. The correct answer should be r/2. The provided key 'a' is likely a typo. Based on physics principles, r/2 is correct. The closest option to r/2 is r/4, but r/2 is the precise answer.