The keystone to what Henry Clay called the American System was?

A. abolishing slavery.
[B. a high tariff.]
C. encouraging British competition.
D. a low tariff
is my answer in brackets correct

Right.

Yes, your answer in brackets is correct. The keystone to what Henry Clay called the American System was a high tariff. To arrive at this answer, you can examine the options presented and eliminate those that are not relevant. Among the options given, only "a high tariff" aligns with Henry Clay's American System. Clay proposed a system that included protective tariffs to promote domestic industry, along with other measures such as a national bank and internal improvements.