The P-T phase diagram for CO2 is shown in the attached figure.
(a) The three phases can coexist at triple point. From the graph, it is at −56.6oC and 5.11 atm.
(b) With the decrease in pressure, both the fusion and boiling point of carbon dioxide will decrease.
(c) For carbon dioxide, the critical temperature is 31.1oC and critical pressure is 73.0 atm. If the temperature of carbon dioxide is more than 31.1oC, it can not be liquefied, however large pressure we may apply.
(d) Carbon dioxide will be (a) a vapour, at =70oC under 1 atm. (b) a solid, at −6oC under 10 atm (c) a liquid, at 15oC under 56 atm.