Final answer:
After 200 years, which is equivalent to four half-lives of 50 years each, 6.25% of the original radioactive atoms remain. The correct answer is D) 6.25%.
Step-by-step explanation:
We can calculate the percentage of the original radioactive atoms remaining after a certain period of time by using the concept of half-lives. In this instance, the isotope has a half-life of 50 years. After one half-life (50 years), 50% of a sample would remain. After two half-lives (100 years), this amount would reduce to 25%. After three half-lives (150 years), it would go down to 12.5%. And after four half-lives (200 years), it would halve again to 6.25%.
Therefore, the correct answer is D) 6.25%, because this is the percentage of radioactive atoms that would remain radioactive after 200 years, equivalent to four half-lives of 50 years each.