$"$Yzx$"$: In this sequence, the child process is created using the fork$()$ function and it executes the code inside the if statement. So$,"$Y$"$ is printed by the child process. Then, the parent process executes the code inside the else statement and prints $"$z$".$ Finally, both the child and parent processes reach the end of the program and print $"$x$".$

$"$zYx$"$: In this sequence, the parent process is created using the fork$()$ function and it executes the code inside the else statement. So$,"$z$"$ is printed by the parent process. Then, the child process executes the code inside the if statement and prints $"$Y$".$ Finally, both the child and parent processes reach the end of the program and print $"$x$".$

$"$zY$"$: In this sequence, the parent process is created using the fork$()$ function and it executes the code inside the else statement. So$,"$z$"$ is printed by the parent process. Then, the child process executes the code inside the if statement and prints $"$Y$".$ The program ends without reaching the final printf statement, so $"$x$"$ is not printed.

The correct answer is: $"$Yzx$","$zYx$","$zY$".$