Correct option is C
The correct answer is
(c) in the prehistoric era have proven.
Explanation:
The error lies in the verb "have proven." The subject of the sentence is "Tracing the origin," which is singular. Hence, the correct verb should be "has" instead of "have."
Corrected sentence:
"Tracing the origin of horse domestication in the prehistoric era
has proven to be an exceedingly difficult task."