Who discovered radioactivity?

For the radioactive nuclei that undergo either $\alpha$ or $\beta$ decay, which one of the following cannot occur?

Which of the following statements is true?

$A$ radioactive element $_{90}X^{238}$ decays into $_{83}Y^{222}$. The number of $\beta$-particles emitted is:

At a specific instant,the emission of a radioactive compound is deflected in a magnetic field. The compound can emit:
$(i)$ Electrons
$(ii)$ Protons
$(iii)$ $He^{2+}$
$(iv)$ Neutrons
The emission at that instant can be:

In $\beta^{-}$ decay, a neutron transforms into a proton within the nucleus according to the equation:
$\text{neutron} \rightarrow \text{proton} + \beta^{-} + x$
In this equation, the particle represented by '$x$' is: