In solids, inter-atomic forces are

The correct option is Combination of (a) and (b).

Let us understand the nature of inter-atomic forces in solids step-by-step:
1. Attractive Forces: When atoms in a solid are separated by a relatively large distance, attractive forces dominate. These attractive forces (such as electrostatic attraction, van der Waals forces, or covalent/metallic bonding forces) tend to pull the atoms closer together to form a stable structure.
2. Repulsive Forces: As the atoms come very close to each other, their outer electron shells begin to overlap. According to the Pauli Exclusion Principle and electrostatic repulsion between the nuclei, a strong repulsive force comes into play, preventing the atoms from collapsing into each other.
3. Equilibrium: In a stable solid, the atoms find an equilibrium position where the net force is zero. At this equilibrium distance, the attractive force exactly balances the repulsive force.
Therefore, the inter-atomic forces in solids are a combination of both attractive and repulsive forces, depending on the distance between the atoms.