Answer:
B. increases the relative concentration of uranium-235 to 30-50% to avoid a nuclear explosion at higher percentages.
Step-by-step explanation:
Uranium enrichment may be defined as a process which is necessary for creating effective nuclear fuel from the mined uranium by mainly increasing the percentage of the uranium-235 that undergoes fission with a thermal neutrons.
The enrichment process removes the unwanted uranium-238, thus making uranium 235 concentration higher. It takes up much work to increase the relative concentration of the uranium 235 to make it 30% to 50% so as to avoid a nuclear explosion at such higher percentages.