Molecules are held together in a crystal by

The correct option is Van der Waal’s attraction.

To understand why this is the correct answer, let us break down the classification of solids and the intermolecular forces holding them together:

1. Molecular crystals (or molecular solids) consist of molecules held together by weak intermolecular forces rather than covalent, ionic, or metallic bonds.
2. These weak intermolecular forces are collectively known as Van der Waals forces (or Van der Waal's attraction).
3. Depending on the nature of the molecules, molecular crystals can be classified as:
    • Non-polar molecular crystals: Held together by weak dispersion forces or London forces (a type of Van der Waals force), such as in solid $I_2$ or dry ice (${\mathrm{CO}}_2$).
    • Polar molecular crystals: Held together by relatively stronger dipole-dipole interactions (also a subclass of Van der Waals forces), such as in solid ${\mathrm{SO}}_2$.
    • Hydrogen-bonded molecular crystals: Held together by hydrogen bonds, such as in ice ($H_{2}O$).

Since the question asks generally about the forces that hold molecules together in a crystal (without specifying polar, non-polar, or hydrogen-bonded), the most comprehensive and general term for these intermolecular forces is Van der Waal’s attraction.