To solve this problem, we need to identify the type of thermodynamic process for each curve in the $P-V$ diagram.$\\$The processes are labeled as I, II, III, and IV. Let's analyze each:$\\$• Process I: This is a vertical line on the $P-V$ diagram, indicating constant volume.$\\$ - This is an isochoric process.$\\$• Process II: This curve is along an isotherm (constant temperature line).$\\$ - This is an isothermal process.$\\$• Process III: This is a horizontal line on the $P-V$ diagram, indicating constant pressure.$\\$ - This is an isobaric process.$\\$• Process IV: This curve is steeper than the isotherms, indicating no heat exchange.$\\$ - This is an adiabatic process.$\\$Now, match the processes with the options:$\\$Column-I:$\\$P. Process I$\rightarrow$c. Isochoric$\\$Q. Process II$\rightarrow$d. Isothermal$\\$R. Process III$\rightarrow$b. Isobaric$\\$S. Process IV$\rightarrow$a. Adiabatic$\\$This corresponds to Option 1:$\\ P \rightarrow$c$, Q \rightarrow$d$, R \rightarrow$b$, S \rightarrow$a$\\$Therefore, the correct option is Option 1.