Apure substance has a triple point in its phase diagram at 216 k and 5.1 atm. this means that:

a at a pressure of one atmosphere the substance can coexist as a solid and vapor.
b at room temperature and one atmosphere pressure the substance is a liquid.
c above the triple point pressure and temperature the substance can exist only as a vapor.
d at a constant temperature of 216 k the substance is a solid at any pressure.
e at a constant pressure of 5.1 atm the substance is a liquid at any temperature.