The correct answer is A) cataracts.
Nubians did not use the Nile for trade because of the cataracts.
The Nile River in North Africa is a navigable river in some parts of the Nile. However, on the part that crosses Nubia, there was a cataract that impeded navigation for the Nubians. Today, that cataract is part of the Nasser Lake. In that region of Nubia and Egypt, there is another cataract that is located in Aswan, Egypt.
Probably, you do not know that the Nile River has six cataracts along the river. One is Egypt (Aswan) and the other five are in Sudan (today, Nubia is part of Sudan).
