Answer
Choice D.
Work Step by Step
Radioactive decay is an exponential process.
After one half-life, 10 years, we are down to one-half the original amount.
After two half-lives, 20 years, we are down to one-quarter the original amount.
After three half-lives, 30 years, we are down to one-eighth the original amount.
After four half-lives, 40 years, we are down to one-sixteenth the original amount.
It cannot be determined when the very last nucleus decays, but it’ll be a lot longer than 40 years for any realistically-sized sample.