If ( X -> Y ) is trivial function means Y is subset of X (X ∩ Y = not-null),
example (sid+sname) -> sid, here X must be SuperKey.
But in question is say (X -> Y) is Non-trivial means X ∩ Y = null , means X is must be not SuperKey.
Given is in 3NF and not in BCNF.
3NF => means (L.H.S = CandidateKey or SuperKey) or (R.H.S = Primary Attribute)
BCNF => means (L.H.S = CandidateKey or SuperKey) and (R.H.S = non Primary Attribute) .
We already know X(LHS) is not superKey , hence to fulfill it is in 3NF and not in BCNF Y(RHS) must be a Prime attribute .Since it also clear that its not in BCNF.
Option A is correct.