For option 3)
Consider a 3NF that is not a BCNF:
This is only possible when a Non Super Key -> Prime attribute
Since every CK is a SK, we can say that this implies:
Non CK (X) -> Prime Attribute (Z)
Say Z is a Prime Attribute because it is a part of CK AZ.
(If Z itself was the entire CK, then even X would be a Super Key and it would not violate BCNF.)
So it is in 3NF, Not BCNF (as assumed) and we are only aware of 1 compound CK AZ (that made Z a Prime Attribute).
Now, since X -> Z therefore, AX -> AZ -> Everything. That means AX is also a CK. That means there has to be at least one more Compound CK for this to be in 3NF and Not BCNF.