Sentence Correction (SC) | Re: Scientists who studied the famous gold field known as Serra

Parallelism

In A, “…not by X but that Y…” doesn’t maintain parallelism.
In B, “…not by X but instead Y…” is not parallel. Moreover, “instead” is redundant here.
In C, “…not by X but Y…” is not parallel. “by” is needed after “but”.
In D, “…not by X but by Y…” is parallel. Moreover, X and Y are parallel too. Hence, D is the best choice.
In E, “…not by X but Y…” is not parallel.

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