Thermodynamic processes are indicated in the following diagram.

Question

$\begin{array}{1 1} (1)P \to a, Q \to c, R \to d, S \to b \\ (2)P \to c, Q \to a, R \to d, S \to b \\ (3)P \to c, Q \to d, R \to b, S \to a \\ (4)P \to d, Q \to b, R \to a, S \to c \end{array}$