A is correct here. The passage tells us that the gold from Senegal was unrefined, but refined gold could be refined to a greater purity than the Senegalese gold. However, merchants preferred to deal in Senegalese gold.
A explains why pretty well, it tells us that the gold from Senegal was standard, while the refined gold varied in its content. This is a pretty strong reason to use the gold that is less pure, if you actually know the amount of pure gold you are getting.
C does the opposite. It tells us that the purity of both the Senegalese and the refined gold was knowable. This makes it harder to understand why merchants would prefer the less pure gold, as it shows it was not more certain how much gold you were actually receiving.
