Answer:
B. A nuclear reaction, because there was a change in the original atom's nucleus
Step-by-step explanation:
A nuclear reaction is the process in which a new nuclides or more than one new nuclides are produced from the collision of two nuclei or a subatomic particle from outside the system and an atomic nucleus
The products of a nuclear reaction must include the transformational change of a nucleus into another nuclide
Therefore, given that the reaction involves the transformation of the nucleus of the uranium-238 nucleus into two nuclides, including; thorium-234 and helium-4, nuclei, the process is a nuclear reaction
uranium-238 → thorium-234 + helium-4.