The most common noble gas obtained in the radioactive decay is

•  The noble gas elements are as follows:

Helium, neon, argon, krypton, xenon and radon.

• The radioactive decay of Radium gives Radon

                          The reaction is as follows:

                                ${}_{88}{}^{226}\mathrm{Ra}\to {}_{{ }_{86}^{222}}\mathrm{Rn}+{}_2{}^4\alpha$

Therefore from the reaction above the most common noble gas obtained in the radioactive decay is  $\mathrm{He}$