Hello,
The answer is option D "reduce the high levels of carbon dioxide emitted into the air".
Reason:
Making nuclear energy requires a lot of carbon dioxide therefore its all going into the air causing the Earth to heat up then causing Global warming and even maybe climate change so scientists have been trying to come up with ways they can make nuclear energy but reduce the amount of carbon dioxide it needs to be made.
If you need anymore help feel free to ask me!
Hope this helps!
~Nonportrit