Conclusion: Predisposition to HBP reflects Western high-salt diets and genes adapted to scarcity of salt
Premise: Black Americans twice as likely to get HBP as White Americans. Westernized Black Africans twice as likely to get HBP as White Africans.
Assumption: ???

Strengthen and Weaken questions often relate to assumptions, but more likely when the question asks you to strengthen or weaken the ARGUMENT itself. In this case, we're asked to strengthen the hypothesis (i.e. conclusion), which means we're likely to bring in new information, so it's unlikely we'll be able to predict the assumption...

(A) The blood pressures of those descended from peoples situated throughout their history in Senegal and Gambia, where salt was always available, are low.
ANSWER: This gives NEW INFORMATION that matches up with the hypothesis (not much salt, no blood pressure problems).

(B) The unusually high salt consumption in certain areas of Africa represents a serious health problem.
PROBLEM: This neither strengthens nor weakens. We want a connection between blood pressure and salt.

(C) Because of their blood pressure levels, most White Africans have markedly decreased their salt consumption.
PROBLEM: This is tricky. Even though it connects blood pressure and salt, it does it in the wrong way. We want to see salt AFFECTING blood pressure. But here we're only told that these people have decreased their salt consumption because of their blood pressure. Has it worked? We don't know.

(D) Blood pressures are low among the Yoruba, who, throughout their history, have been situated far inland from sources of sea salt and far south of Saharan salt mines.
PROBLEM: This is less tricky than it looks. This just says a tribe without salt doesn't have high blood pressure. This would strengthen the conclusion that salt causes high blood pressure. But we already know that. We want to know if there's a genetic link that causes those with a history of low-salt diets to develop high blood pressure when moved to the West. This doesn't address the West at all.

(E) No significant differences in salt metabolism have been found between those people who have had salt available throughout their history and those who have not.
PROBLEM: You could argue this has no effect (what do we care about salt metabolism?) or weakens, because it's breaking the link between history and blood pressure that the conclusion wants to make.