Based on nuclear stability, what is the symbol for the most likely product nuclide when iodine-123 undergoes decay?

Answer is: Te (tellurium).
During electron capture, iodine-123 form the nearly-stable nuclide tellurium-123.
In beta plus decay (atomic number Z is decreased by one), a proton is converted to a neutron and positron and an electron neutrino, so mass number does not change.
Iodine-123 is a radioactive isotope of iodine used in nuclear medicine imaging.