Answer
Sn-112
Work Step by Step
The number of protons is equal to the atomic number. Sm has an atomic number of 62, so we look for the option with 62 neutrons, for we are looking for what option has the same number of neutrons as Sm has protons. The number of neutrons is equal to the mass number minus the number of protons. Recalling this, we find that Sn-112 is the right option.