This is inference question that we can infer the right answer choice from the passage.
In the first paragraph, line 10 states that modern scholars should have been interested more in analyzing similarities and differences between serfdom and slavery. Tocqueville is introduced as another figure who were likely to be interested in this issue but not exploited into this, despite his "recognition" of this issue and interest in United States Slavery (7)
Let's go to the answer choices.
A) Tocqueillve, according to 4-6, “recognized” the comparability, but the author never states that he did anything about it. And he was actually interested in “United States Slavery” not BOTH according to line 7.
B) T "never compared their system of servitude" (5-6) supports this. (Correct)
C) “inability” is not true because he never tried.
D) He didn’t even try doing so, thus overestimation itself could not occur.
E) This is only about T, not related with other historians.