First you bring the chicken over. Then you go back without the chicken. Then you bring the chicken food over and take the chicken back. Then bring the fox over leave him there with the chicken food and go get the chicken!
The correct password is “Seven”.
“Twelve” consists of 6 letters T w e l v e hence the password is “Six”. “Six” consists of 3 letters S i x hence the password is “Three”. “Sixteen” consists of 7 letters S i x t e e n hence the correct password should be “Seven”.