Using the phase diagram for CO2 what phase is carbon dioxide in at -20C and 1 ATM pressure?

The triple point of Carbon Dioxide is 216.55 K (−56.60 °C) and 517 kPa (5.10 atm). Since that puts the pressure (1 atm) below the triple point pressure (5.1 atm) we are only concerned with the where the solid/vapor equilibrium line falls relative to the temperature. At 1 atm, the sublimation temperature of Carbon Dioxide is -78.5 °C - considerably below -20 °C so that puts the Carbon Dioxide firmly in the vapor region of the phase diagram.