Final answer:
The percentages of ²³⁵U and ²³⁸U in natural uranium when the Earth formed 4.5×10⁹ years ago were 0.7202% and 99.25%, respectively. The correct option is c).
Step-by-step explanation:
Natural uranium is composed of 0.7200% ²³⁵U and 99.27% ²³⁸U. To determine the percentages of these isotopes when the Earth formed 4.5×10⁹ years ago, we need to consider the half-lives of the isotopes. The half-life of ²³⁵U is 7.04×10⁸ years, while the half-life of ²³⁸U is much longer at 4.47×10⁹ years.
Since the Earth formed 4.5×10⁹ years ago, the proportion of ²³⁵U would have decreased due to its shorter half-life. Therefore, the percentage of ²³⁵U in natural uranium when the Earth formed would have been lower than 0.7200%.
On the other hand, the longer half-life of ²³⁸U means its percentage in natural uranium would have remained relatively stable over time, so it would still be close to 99.27% when the Earth formed.
Therefore, the correct answer is c) 0.7202%, 99.25%.