When an element undergoes alpha decay, it loses an alpha particle, which consists of 2 protons and 2 neutrons. Therefore, the atomic number decreases by 2, and the mass number decreases by 4.
For the element \( ^{234}_{90}Th \) (Thorium), the calculations are as follows:
- Initial atomic number: 90 (Thorium)
- Final atomic number after alpha decay: 90 - 2 = 88
- Initial mass number: 234
- Final mass number after alpha decay: 234 - 4 = 230
Looking at the periodic table, the element with atomic number 88 is Radium (Ra).
Thus, after alpha decay, \( ^{234}{90}Th \) changes into \( ^{230}{88}Ra \) (Radium).
The correct answer is c) Radium (Ra).