There are two answers for this question, BUT, I say D is the best of these two
A. Their trading boats were unable to go past them;
D. The Egyptians used them as a defense against the Kush's armies;
The cataracts on the river Nile are part of the river where the waters are shallow, and there's also lots of rocks that are very close to the surface of the water. This makes them unsuitable for boats and for the Kush that was a big problem. The cataracts were limiting Kush to only perform trade and military actions on land, and Egypt used that advantage to the maximum.
The Nile cataracts can be used for sailing with boats but only when there's flood, so that is only in a very short period of time, and not to mention that when there's flood the waters are very unpredictable and can be fatal.