In P versus V graph, the horizontal line is found in which ____...

From the graph, at point (A, \mathrm{CO}_2) exists as a gas, As pressure is increased, the volume of the gas decreases along the curve $A B$. At $B$ liquefaction of the gas starts. Hence, volume decreases rapidly along $B C$ because liquid has much less volume than the gas. At point $C$, liquefaction is complete. Thus, along horizontal line, gas converts into liquid.