GMAT Sentence Correction (SC) | Re: At a conference on the origins of language, the complexity of tools

(A) were said to suggest a level of human development inconceivable unless they had language--> Complexity is singular. Hence, were is incorrect.

(B) were said to suggest that without language their level of human development was not conceivable --> Complexity is singular. Hence, were is incorrect.

(C) was said to be suggestive of a level of human development not conceivable without language --> Said to be suggestive is wordy and awkward.
(D) was said to suggest a level of human development
...

---