From Navathe 5E, page 364

A FD $X\rightarrow Y$ in a relation schema R is a **transitive dependency** i__f there is a set of attributes Z that is neither a candidate key nor a subset of any key__ of R and both $X\rightarrow Z$ and $Z\rightarrow Y$ hold. This implies that Z must be a non-prime attribute.

Option C is given as "A prime attribute can be transitively dependent on a key". This means there exist two FDs like..

1) $KEY\rightarrow NON PRIME$

2) $NON PRIME\rightarrow PRIME$

(here $KEY\rightarrow PRIME$ is transitive dependancy as said in option C and intermediate is non prime because Z must be non prime according to the definition)

This two FDs are in 3NF because LHS of FD1 is superkey and RHS of FD2 is Prime, But it is not in BCNF because LHS of FD2 is not a super key.