Which of the following cannot be the order of execution and completion of processes? a) P1, P2 , P3, P4, P6, P5 b) P1, P2, P4 P3, P5, P6 c) P1, P3 P5, P2, P4, P6 d) P1, P3, P2 P5, P4, P6

The correct answer is option a) P1, P2, P3 P4, P6, P5.<br /><br />In order for processes to be executed and completed in a specific order, each process must be completed before the next one can begin. In option a), the processes P4 and P6 are executed before P5, which means that P5 cannot be completed before P4 and P6 are finished. Therefore, option a) cannot be the order of execution and completion of processes.