When radium-226 (atomic number-88) decays by emitting an alpha particle it becomes
a. polonium-222
c. radium-222
b. polonium-224
d. radon-222

d. Radon-222

Explanation:

²²⁶₈₈Ra → ²²²₈₆Rn  + ⁴₂He

Alpha particle is a helium nucleus with mass number 4 and atomic number 2. According to the law of conversation of mass, the sum of the mass number and atomic number must be equal on both side of the reaction.  

Since the mass number of Ra is 226 and that of He is 4. The mass number of the unknown element must be 226 - 4 = 222.  

Since the atomic number of Ra is 88 and that of He is 2. The atomic number of the unknown element must be 88 - 2 = 86.  

Now looking in the periodic table Radon is the only element with atomic number 86.