Your answer is B.
Zimbabwe was at its peak during the 13th and 14th centuries by thriving on cattle herding and gold mining specifically, which created strong trade routes with all the neighboring countries.
However, entering the 15th centuries, the rivers that at one point seemed somewhat filled with gold flakes and nuggets ran out of Zimbabwe's prime export.
A. is wrong because Timbuktu was the central region for salt mining and C. is wrong because the agricultural concern was with the cattle, and finally D. is incorrect because the Portuguese are the ones who for a time conquered a great portion of Zimbabwe.
Your answer is B.