Answer
Please see the work below.
Work Step by Step
We know that
a.$Sn^{18}_{50}$ has atomic number 50. It has 50 protons and 68 neutrons. As the atomic number is less than 83 and it has an even number of protons and neutrons, hence it is stable.
b. $As^{76}_{33}$ has atomic number 33 and it has 33 protons and 43 neutrons. It has radio active isotopes.
c. $Ac^{227}_{89}$ has 89 atomic number and it is radioactive.
