23) B

Mass number only decreases by alpha decay. After each alpha decay, the mass number decreases by 4.

Decrease in mass number = 244 – 220 = 24
Number of alpha decays = 24 ÷ 4 = 6

After 6 alpha decays, the atomic number will decrease by 12.
So atomic number would decrease to 82 (94 – 12 = 82)

After each beta decay, the atomic number increases by 1. We know the atomic number of radon is 86. So there must be 4 beta decays to increase the atomic number from 82 to 86.

After 6 alpha decays and 4 beta decays, plutonium-244 becomes radon-220.

So the answer is B.