Answer:
A reaction that joins the nuclei of two atoms into one  
Explanation:
Think of the words "nuclear fusion".
Nuclear = "relating to a nucleus"
 Fusion = "joining"
So, nuclear fusion is the joining of two nuclei into one.
A and B are wrong, because chemical reactions involve electrons.
D is wrong, because the splitting of a nucleus is fission.
 
        
             
        
        
        
Answer:
mitochondria
Explanation:
Remember that mitochondria is the powerhouse of the cell
ATP is considered energy