B

Let's analyze the processes depicted in the p-V diagram:
1. **Process ac:** The volume changes while the pressure remains constant. This corresponds to fixed pressure hence it is a process with **varying volume**.
2. **Process cb:** In this process, the volume remains constant while the pressure decreases, indicating a change in volume under constant pressure, identifying it as a path with **varying pressure**.
3. **Process ab:** Throughout this process, both pressure and volume change, suggesting that the temperature is also changing, hence it is a process with **varying temperature**.
4. **Process db:** As the gas transitions from state d to b, the internal energy typically decreases if the volume decreases at constant pressure, classifying it as a process with a **decrease in internal energy**.
Therefore, this results in the matching: (i) Varying Pressure [B], (ii) Varying Volume [A], (iii) Varying Temp. [C], (iv) Decrease in internal Energy [D]. Thus, the correct answer for the matching is **B**.