Used fuel assemblies are typically considered a. low-level waste b. uranium mill tailing c. high-level waste d. all of the above

c. high-level waste

Explanation:

The used or spent fuel comes from the nuclear reactor that can be no longer efficient to be used again for the purpose of generating electricity, because the efficiency to conduct fission reaction by such fuel is low. But still it may be thermally hot, highly radioactive and harmful. It comes under the category of high level wastes. The high level wastes are those wastes that are produced as a byproduct of the nuclear reactors.

i am positive it is c. condensing but i may be wrong

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.