Dry ice is the solid state of carbon dioxide ($CO_2$). At atmospheric pressure, it sublimes directly from solid to gas at $-78.5^\circ\text{C}$ ($-109.3^\circ\text{F}$) without passing through a liquid phase, absorbing $571\text{ kJ/kg}$ of latent heat, making it an ideal expendable refrigerant in food transport.

Option analysis:

• Solid water: Conventional water ice, melts to liquid at $0^\circ\text{C}$.
• Solid Freon-12: Halocarbon refrigerant, not called dry ice.
• Solid nitrogen: Nitrogen freezes at $-210^\circ\text{C}$, rarely used in food.
• Solid carbon dioxide: Correct, standard scientific identity of dry ice.