Question

during nuclear fission and fusion, what is matter that seems to disappear actually converted into?
mass
energy
volume
nuclei

Explanation:

In nuclear fission and fusion, according to Einstein's mass - energy equivalence principle ($E = mc^{2}$), the matter that appears to disappear is actually converted into energy. Mass is what is being converted, not the end product. Volume is a measure of space occupied and not related to the conversion here. Nuclei are involved in the fission and fusion processes but the "disappearing" matter is converted to energy.

Answer:

B. energy