"Dishonourable" means lacking honor, which is the opposite of what is being appreciated here.
"Commendable" means deserving praise or approval, which perfectly matches the context of appreciating the staff's effort.
"Contemptible" means deserving contempt, which is inappropriate here.
"Reprehensible" means deserving strong criticism, which contradicts the praise.
Thus, "Commendable" is the correct choice.