Among the following which is the best description of water in the solid phase?

In the solid state (ice), the fundamental repeating units are discrete $$H_2O$$ molecules.

These $$H_2O$$ molecules are held together in the lattice primarily by hydrogen bonds, which are a type of strong intermolecular force but NOT true covalent bonds extending throughout the crystal.

Classification of crystalline solids:
• Covalent (network) solids - a giant three-dimensional network of atoms joined by covalent bonds (e.g., diamond, $$SiO_2$$).
• Ionic solids - lattice points occupied by cations and anions held by electrostatic forces (e.g., $$NaCl$$).
• Molecular solids - lattice points occupied by neutral molecules held by van der Waals forces, dipole-dipole forces or hydrogen bonds (e.g., $$CO_2$$ (dry ice), ice).
• Metallic solids - lattice of positive ions in a sea of delocalised electrons.

Because ice consists of neutral $$H_2O$$ molecules held by hydrogen bonding, it fits the definition of a molecular solid.

The other options do not apply:
• Option A (covalent solid) and D (network solid) require an extended covalent network, which ice does not possess.
• Option C (ionic solid) would require ions such as $$Na^+$$, $$Cl^-$$, etc., which are absent in pure ice.

Hence, the best description is:

Option B which is: Molecular solid