Electrostatic attraction between the ions
A salt crystal consists of Na⁺ and Cl⁻ ions stacked together in a crystal.
The ions have opposite charges, so they are electrostatically attracted to each other.
Because the ions are so close together, the attractive forces are quite large.
Option (b) is the correct answer.
An atom consists of three sub-atomic particles, that is, there are protons, neutrons, and electrons in an atom.
Protons are positively charged, neutrons have no charge and electrons have a negative charge.
It is known that opposite charges attract each other. And a salt contains atoms of two or more different elements.
Therefore, there occurs attraction between a proton and an electron which is responsible for the force of attraction between the particles in a salt crystal.
The interaction responsible for the force of attraction between particles in a salt crystal is that between a proton and an 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 could be the electromagnetic interaction if you're talking about the 4 fundamental interactions, or ionic interaction, or electrostatic interaction.