Answer:
Uranium-238 (U-238) takes longer to decay radioactively compared to plutonium-239 (Pu-239).
Uranium-238 has a half-life of about 4.5 billion years, which means that it takes 4.5 billion years for half of a sample of U-238 to decay. On the other hand, plutonium-239 has a shorter half-life of about 24,100 years.
Therefore, uranium-238 is considered a more stable isotope compared to plutonium-239 in terms of radioactive decay.