The correct option is Snow.
Explanation
Precipitation occurs in various forms depending on the atmospheric temperature and the process of condensation. When water vapor condenses directly into a solid state (deposition) without passing through the liquid phase, it forms ice crystals with specific geometric structures.
- Rain Rain: This is precipitation in the form of liquid water droplets. It does not possess a crystalline structure.
- Snow Snow: Correct. Snow is formed when water vapor in clouds converts directly into ice crystals at temperatures below freezing. Due to the specific hydrogen bonding geometry of water molecules ($H_2O$), these ice crystals naturally arrange themselves into a hexagonal (six-sided) lattice structure.
- Sleet Sleet: Sleet consists of frozen raindrops or refrozen melted snow. It typically takes the form of small, translucent ice pellets rather than delicate, hexagonal crystals.
- Hail Hail: Hail consists of hard pellets or lumps of ice produced in cumulonimbus clouds. Hailstones grow through the accretion of supercooled water droplets, resulting in a layered, onion-like structure rather than a single hexagonal crystal form.
Key Takeaway: Snowflakes are aggregates of ice crystals that invariably form in a hexagonal shape due to the molecular structure of water when it freezes.