Sentence Correction

C seems to be the best choice but rules say that the commas are not required in C

leo wrote:Over five thousand years ago, archaeologists have discovered that Mesopotamian cooks were preparing chicken with coriander and tamarind, just as Egyptian cooks are still doing today.
(A) Over five thousand years ago, archaeologists have discovered that
(B) Over five thousand years ago, archaeologists had discovered that
(C) Archaeologists have discovered that, over five thousand years ago,
(D) Over five thousand years ago, as archaeologists have discovered it,
(E) Archaeologists have discovered, over five thousand years ago, that

Is answer A,B or C? Please explain.

It's definitely (C).

If we use the "ignore what's inside the commas" rule, we get:

Archaeologists have discovered that Mesopotamian cooks were preparing chicken with coriander and tamarind, just as Egyptian cooks are still doing today.

which is a perfectly acceptable sentence. The parenthetical comment between the commas is just bonus information that's not required for the sentence to make sense. It's also a parenthetical comment in the original sentence (introductory descriptive phrase), so putting it between commas in the middle of the sentence doesn't change the meaning.

"as archaelogists have discovered it" is a horrible phrase and enough on its own to warrant eliminating (d). The pronoun "it" has no good antecedent.