1 attribute candidate key: None (Every attribute has 2 tuples same)
2 attribute candidate key: (Last Name, Rank), (Last Name, Room Number), (Last Name, Shift), (Rank, Shift), (Room Number, Shift).
(Rank, Room Number) cannot be a 2 attribute candidate key because (Custodian,20) is there twice. So, if we add one more attribute to it, like (Rank, Room Number, Last Name) or (Rank, Room Number, Shift), it becomes a Superkey.
So, only 5 candidate keys.