The correct answer is C.
The answer is C, as when placed in the sentence it says:
"Morgan was talented not only as a musician but also "as a composer".
A, wouldn't make any sense, as the sentence is only saying what she is talented as. Make sure to use the same structure as the sentence, such as the repetitive, "as a".
B, is not right, as it is stating that she could compose songs, but its not stating the talent, this statement was inferring that she could compose song, but it seems like it wasn't clear enough. Same applies with D.
So the correct answer is C.