Final answer:
The energy released in the fusion of deuterium nuclei is in the form of nuclear energy, resulting from the transformation of mass into energy.
Step-by-step explanation:
The energy released in the fusion of two almost stationary deuterium nuclei H2 to form a tritium nucleus H3 and a proton is in the form of nuclear energy. Nuclear fusion involves the conversion of a small amount of mass into energy, as described by Einstein's equation E = mc2. The release of 6.4 x 10-13 J of energy in this fusion reaction indicates the transformation of the mass lost into energy.