(a) No. CO2 at 1 atm pressure and –60°C is vapour. If it is compressed isothermally i.e. when pressure is increased without changing the temperature, it will go to solid phase directly without going through the liquid phase.
(b) CO2 at 4 atm pressure and at room temperature (say 25°C) is vapour. If it is cooled at constant pressure, it will again condense to solid without going through the liquid phase (the horizontal line through the initial point intersects only the sublimation curve III).
(c) CO2 at 10 atm pressure and at –65°C is solid. As CO2 is heated at constant pressure, it will go to liquid phase and then to the vapour phase. It is because, the horizontal line through the initial point intersects both the fusion and vapourisation curves. The fusion and boiling points can be known from the points, where the horizontal line at P-T diagram at 10 atm (initial point) intersects the respective curves.
(d) It will not exhibit any clear phase transition to the liquid phase. However, CO2 gas will depart more and more from the ideal gas behaviour as its pressure increases.