Network solid, among the following, is :

1. SO2(solid)

2. I2

3. Diamond

4. H2O(ice)

HINT: Molecules or atoms are held together by covalent bonds.
Explanation:
A network solid or covalent network solid is a chemical compound in which the atoms are bonded by covalent bonds in a continuous network extending throughout the material. 
Diamond is a giant molecule in which constituent atoms are held together by a covalent bond. Hence, this is a network solid.