This is "detail" question where we find direct reference to what the author is asking us.
line 13-20 states that some were put off by “forbidding political differences”
Let's go to the answer choice.
A) Basically a paraphrase of line 13-20. This is it. (Correct)
B) Unsupported. Although this difference is highlighted, this is not presented as the REASON why historians got discouraged.
C) Way Off. Tocqueville's influence on other historians is not discussed. Out of scope.
D) This is just a fact presented from the passage, not what discouraged historians.
E) Out of scope. "Comprehensiveness of Kolchin's own work" is not discussed in the passage at all.