Final answer:
In the nuclear reaction ^238U92 → ^234Th90 + ?, alpha radiation is emitted. This is seen as the emission of an alpha particle, which leads to a decrease in both the atomic and mass numbers of the uranium atom.Therefore, the correct answer is A) alpha radiation.
Step-by-step explanation:
The nuclear reaction provided, ^238U92 → ^234Th90 + 4n, is a spontaneous fission process that occurs in the decay of uranium-238 (^238U). During this process, uranium-238 decays into thorium-234 (^234Th) by releasing four neutrons (n).
The type of radiation emitted in this nuclear reaction is not beta or gamma radiation. Beta radiation refers to the emission of high-energy electrons or positrons, while gamma radiation refers to high-energy electromagnetic radiation. Instead, this reaction releases alpha radiation, which consists of helium nuclei (alpha particles) with a mass of four and a charge of two.
Alpha particles are relatively large and heavily ionizing, making them easily stopped by a few centimeters of air or a sheet of paper. Therefore, the correct answer is A) alpha radiation.