Which type of radioactive decay results in no change in mass number and atomic number for the starting nucleus?

Gamma Rays

Explanation:

For the starting nucleus, the gamma rays is the type of radioactive decay results in no change in mass number and atomic number. This is only due to the fact that gamma rays don't have any mass and also don't have any kind of electric charge. So after decaying it shows no sign of change in atomic number because neither the mass has changed nor the charge.