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 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), P4 and P6 are executed before P5, which is not possible because P5 must be completed before P4 and P6 can begin. Therefore, option a) cannot be the order of execution and completion of processes.