Answer
alpha
Work Step by Step
Let the particle be $\,^{A}_{Z}X$
We can write the nuclear equation as:
$^{214}_{90}Th\rightarrow \,\,^{210}_{88}Ra+\,^{A}_{Z}X$
Sum of charges and masses should be equal on both sides. This implies that
$214=210+A$ or $A=4$ and
$90=88+Z$ or $Z=2$
The particle with mass 4 and charge 2 is produced. The particle is alpha.
