(B) says, "In the early nineteenth century, colonies that employed forced labor were still economically viable."
We're looking for a contention that Eltis argues against. The problem with (B) is that we know that Eltis believes that Williams underestimated the economic viability of the British colonies employing forced labor in the early 1800s (lines 54-57), so Eltis must have believed that they were at least a little viable. Thus, (B) is incorrect, as Eltis did not argue against what (B) is saying.
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