Answer: option B) proton-electron.
The particles inside a salt crystal are ions. The electrostatic attraction between the positive ions are the negative ions is responsible for the crystal structure. In last stay, the protons are responsible for the positive charge and the electrons for the negative charge. Then you can say that the interaction proton-electron is responsible for the force of attraction between the particles in a salt crystal.
The answer is liter hope this helped :)
Http://www.wasatch.edu/cms/lib/UT01000315/Centricity/Domain/647/dilution%20problems%20answers.pdf it would be question 5 if you can read it.
Ill try to help you find the answer