Candidate key:- must be able to uniquely determine every other attribute. i.e., whenever a candidate key value is repeated, all other values must repeat or in a set, candidate key cannot be repeated.
option
a:- Last Name cannot be key as smith value are repeated.
b.Room:- 33 value is repeated
c.shift:- morning repeated
d. Composite key Room+shift => Every tuple is now unique
234+morning
33+afternoon
33+evening
222+morning.
so Option D is right