Which net change occurs in a nuclear fusion
reaction?
(1) ionic bonds are broken.
(2) ionic bonds are formed.
(3) energy is converted to mass.
(4) mass is converted to energy.

Correct  (4) Mass is converted to energy.

Reason: 
In a nuclear fusion reaction, two small nuclei are fused to formed a relatively heavy nuclei. The mass of the daughter nuclear is less as compared to parent nuclei. This difference is mass is converted into energy. The energy liberated during this process can be estimated by the Einstein's equation i.e. E = m.