Commemorative plaques cast from brass are a characteristic art form of

The author concludes that "it is unlikely that Benin’s knowledge of brass casting derived from the Portuguese." Since we need to strengthen the argument, let's make sure we understand the author's logic:

• "Commemorative plaques cast from brass are a characteristic art form of the Benin culture of West Africa." - This background information tells us that cast-brass plaques are a typical form of art in Benin culture.
• "The oldest surviving plaques date to the 1400s." - These plaques have been a part of Benin culture since at least the 1400s (maybe earlier).
• "Some scholars hypothesize that brass-casting techniques were introduced by the Portuguese, who came to Benin in 1485 A.D." - How did the people of Benin learn brass-casting techniques? Perhaps from the Portuguese. This hypothesis is consistent with the date evidence (the earliest known plaques date to the 1400s, and the Portuguese came to Benin in the 1400s).
• However, "Portuguese records of that expedition mention cast-brass jewelry sent to Benin’s king from neighboring Ife." - When the Portuguese first showed up, Benin already had cast-brass jewelry from Ife. This means that Benin had already been introduced to cast-brass prior to the arrival of the Portugal. This evidence seems to hurt the hypothesis of some scholars. The author thus concludes that Benin's knowledge of brass casting probably did not derive from (or "originate from") the Portuguese.

We need something that strengthens the author's argument (not the scholar's hypothesis):

Notice that (A) does NOT say, "Portuguese records indicate that the expedition did not include metalworkers." (A) says that the records don't indicate either way. So perhaps the expedition included metalworkers and perhaps it did not. Although this leaves open the possibility that the expedition did not include metalworkers, the evidence in (A) is not very strong. Let's look for something better.

The evidence tells us that Benin had cast-brass objects from Ife before they encountered the Portuguese. This suggests that the people of Benin may have learned about cast-brass from Ife, not from the Portuguese.

But what if the people of Ife learned about brass casting from the Portuguese? In that case, any knowledge about brass casting transferred from Ife to Benin actually originated (or derived) from the Portuguese. In other words, if the Portuguese taught the people of Ife about brass casting and then the people of Ife taught the people of Benin about brass casting, then Benin's knowledge of brass casting derived from the Portuguese (even though it was not transferred directly from the Portuguese to Benin).

This would obviously hurt the argument. Choice (B) eliminates this possibility and thus strengthens the argument. Hang on to (B).

All that matters is that the Portuguese knew how to cast brass. We don't care how the Portuguese used cast brass. Even if the Portuguese did not make cast brass commemorative plaques, they still could have taught the people of Benin how to cast brass. Thus, (C) does not strengthen the argument and can be eliminated.

This statement doesn't tell us anything about the origin of the artists' knowledge of brass casting. This statement could be true whether their knowledge derived from the Portuguese or from the people of Ife. Thus, (D) can be eliminated.

This simply tells us that Benin had one of the materials needed to make brass. As with (D), this doesn't tell us anything about the origin of the artists' knowledge of brass casting. Eliminate (E).

(B) is the best answer.