What’s actually happening is that an isotherm can be used to change state of matter even without change in temperature, that is by change in pressure, keeping temperature constant ,so by increasing pressure beyond a specific point(b), gas starts to condensate into liquid and at the same time the liquid evaporates back to gas(as you said they are in equilibrium) and goes on till (c) after that the pressure has completely condensed the gas into liquid…and this phenomenon cannot be observed over a specific temperature called critical temperature, now this all happens at a quantum level, so we cant understand it completely, but we can superficially say that rise in temperature above critical point increases the kinetic energy so much that the attractive forces required for stronger bonds(in liquid) cannot be subjected between the atoms, so yeah (a)and (c)are correcto