Which of the following is a radioactive element?

If uranium (mass number $238$ and atomic number $92$) emits an $\alpha$-particle,what are the mass number and atomic number of the product?

The radionuclide $_{90}^{234}Th$ undergoes two successive $\beta$-decays followed by one $\alpha$-decay. The atomic number and the mass number respectively of the resulting radionuclide are

Decrease in atomic number is observed during

During $\beta$-decay:

$_{11}^{23}Na$ is the more stable isotope of $Na$. Find out the process by which $_{11}^{24}Na$ can undergo radioactive decay.