The correct answer is C. Conversion of mass into energy.
In an atomic explosion, a small amount of mass is converted into a large amount of energy. This is according to Einstein’s famous equation, $E = mc^2$, which states that energy and mass are equivalent and can be converted into each other. The amount of energy released in an atomic explosion is enormous, and can be devastating.
Option A is incorrect because chemical energy is not converted into heat energy in an atomic explosion. Chemical energy is the energy stored in the bonds between atoms, and is released when those bonds are broken. In an atomic explosion, the bonds between atoms are not broken, but rather the atoms themselves are split apart. This process is called nuclear fission.
Option B is incorrect because mechanical energy is not converted into nuclear energy in an atomic explosion. Mechanical energy is the energy of motion, and is released when an object moves. In an atomic explosion, there is no motion of objects, but rather the splitting of atoms.
Option D is incorrect because neutrons are not converted into protons in an atomic explosion. Neutrons are subatomic particles that have no charge, while protons are subatomic particles that have a positive charge. In an atomic explosion, neutrons are not converted into protons, but rather the splitting of atoms releases neutrons.
I hope this explanation is helpful. Please let me know if you have any other questions.