Step-by-step explanation:
If ice is heated below the triple point of pressure of ice then it will directly convert to water vapor. This process is called sublimation.
In these kind of questions pressure is generally kept constant. So, make a constant pressure line which is horizontal line in this graph. Now when something is heated then its temperature increases. So, move from left side to right side on the constant pressure line.
In this case, on moving along the constant pressure line is below the triple point, the first encountered line is line of sublimation ahead of which is vapor phase. So if you give heat to ice at a pressure below constant pressure line than it will directly convert to water vapor.