In studying pre-industrial Europe, these historians have focused on

In studying pre-industrial Europe, these historians have focused on only the 2 or 3 percent of the population that comprised the political and social elite, but they have generally overlooked the rest, thereby failing to understand European civilization as a whole.

Things to notice before moving on to the options :
1) but they have generally....... pronoun 'they' must refer to an antecedent , which is plural.
2) parallelism eg. Scientists have studied the climate, but they have overlooked the global warming.

(A) In studying pre-industrial Europe, these historians have focused on-- seems correct .. let's save it for the end.

(B) Studies of pre-industrial Europe by these historians have been focused on.. 'they' can refer to the studies as well as historians. And also, studies cannot 'fail' to understand the European civilization, but the historians are failing to understand European civilization.

(C) Studying pre-industrial Europe, these historians have had as a focus
(D) These historical studies of pre-industrial Europe have had as a focus
(E) In their study of pre-industrial Europe, historians have been focusing on - this option clearly breaks the parallelism as mentioned above.

In studying pre-industrial Europe, these historians have focused on only the 2 or 3 percent of the population that comprised the political and social elite, but they have generally overlooked the rest, thereby failing to understand European civilization as a whole.

In studying pre-industrial Europe, these historians have focused on only the 2 or 3 percent of the population that comprised the political and social elite, but they have generally overlooked the rest, thereby failing to understand European civilization as a whole.

(A) In studying pre-industrial Europe, these historians have focused on
(correct, historians is the subject)
(B) Studies of pre-industrial Europe by these historians have been focused on
(passive voice error)
(C) Studying pre-industrial Europe, these historians have had as a focus
(past perfect error)
(D) These historical studies of pre-industrial Europe have had as a focus
(past perfect error)
(E) In their study of pre-industrial Europe, historians have been focusing on
(present perfect progressive error)

Historians have... But they have... Parallel only in option 'A'

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