An unstable nucleus loses the most mass if the nucleus emits

(1) an alpha particle (3) a positron

(2) a beta particle (4) a gamma ray

The correct answer is option 1. An unstable nucleus loses the most mass if the nucleus emits an alpha particle. This type of electromagnetic radiation has the largest mass among the radioactive particles. When this particle is emitted from a nucleus, it loses two protons and two neutrons therefore large amount of mass is lost.