Answer:
I believe the mistake in word choice to be:
C. implied
Explanation:
Extant means remaining, or surviving. It makes sense to say there is a remaining, an extant secondary literature.
To comprise means to be made up of, to consist of something. It also makes sense to say the secondary literature consists of, comprises several studies.
To imply means to suggest something. I believe the appropriate verb to be used here should be "to infer". To infer means to conclude something after due reasoning. It would be better if we said that, from the that literature, it can be inferred, concluded, that the definition lies in the middle.
Finally, "to lie somewhere in the middle" is a correct expression, and there is no reason to change it.
