Intermolecular forces are the forces of attraction or repulsion between molecules. These forces play a crucial role in determining the physical and chemical properties of substances. There are several types of intermolecular forces, such as van der Waals forces, dipole-dipole interactions, and hydrogen bonding.

Strong intermolecular forces are typically observed between solid molecules. Here's why:

1. Nature of solid molecules:
- Solid molecules are closely packed and have strong bonding between them.
- The atoms or molecules in a solid are held together by strong covalent or ionic bonds.
- These strong bonds require a significant amount of energy to break, resulting in a high melting and boiling point.

2. Van der Waals forces:
- Van der Waals forces are the weakest type of intermolecular force. They arise due to temporary fluctuations in electron distribution, resulting in temporary dipoles.
- In solids, the molecules are closely packed, allowing for a greater number of interactions between these temporary dipoles.
- As a result, the overall van der Waals forces between solid molecules are stronger than those between gas or liquid molecules.

3. Dipole-dipole interactions:
- Dipole-dipole interactions occur between polar molecules, where the positive end of one molecule attracts the negative end of another molecule.
- In solids, the molecules are arranged in a specific pattern, leading to a greater number of dipole-dipole interactions.
- This arrangement and alignment of polar molecules enhance the strength of these intermolecular forces.

4. Hydrogen bonding:
- Hydrogen bonding is a special type of dipole-dipole interaction that occurs when hydrogen is bonded to highly electronegative elements like oxygen, nitrogen, or fluorine.
- Solid molecules containing hydrogen bonding, such as water (H2O) or ammonia (NH3), have strong intermolecular forces due to the presence of multiple hydrogen bonds.
- These strong hydrogen bonds contribute to the high boiling and melting points of solid compounds.

Therefore, the correct answer is option 'b', solid molecules. Solid molecules have strong intermolecular forces due to the nature of their bonding, a greater number of van der Waals forces, dipole-dipole interactions, and hydrogen bonding.