Answer:
A: Alpha decay.
Explanation:
The transmutation of ²³⁸U is a nuclear reaction that yields ²³⁴Th and an alpha particle.
Now, Alpha decay is a radioactive decay whereby an atomic nucleus emits an alpha particle which then changes into an atom with a mass number that is decreased by four and atomic number decreased by 2.
In this case, mass number is reduced from 238 to 234. Thus, it's an alpha decay.
