Well the knowing what the answer choices mean would be most helpful for this problem:
a. The definition of "Fusion" in general means bringing things together into one unit. In biology, Fusion sometimes refers to cellular fusion, the bringing of two or more uninuclear (cells with one nucleus) together to form multinuclear cells.
B. Fission is almost the opposite of "fusion" in that it is the action of dividing or splitting into two or more parts. This sounds like fission of a heavy nucleus to me.
c. molecular blending forms polymers. So this choice is incorrect.
d. atomic rebonding- when atoms on different molecules combine to form diatomic ions.
hope this helped!