Though research remains to be done into the reasons why the Civil War was triggered, scholars do not regard slavery to be the sole cause.


A .Though research remains to be done into the reasons why the Civil War was triggered, scholars do not regard slavery to be the sole cause.
--> first part is wordy and awkward
B. Though research remains to be done into what triggered the Civil War, scholars do not regard slavery as the sole cause.
C. Though the reasons that triggered the Civil War remain to be researched, slavery is not regarded by scholars to be the sole cause.
--> passive construction B gives a good active construction

D. Despite research remaining into the reasons why the Civil War was triggered, scholars do not regard slavery as the sole cause.
--> first part is wordy and B represents it with brevity. Also "Despite .....", construction is awkward.

E. Scholars do not regard slavery as the sole cause of the Civil War, though the reasons for it being triggered remain to be researched. --> ambiguous "it"

A/C - idiom regard x as y
C - reasons that triggered the Civil War - illogical, passive and redundant. Similar error in A as well reasons why the Civil was triggered.
D - research remaining into the reasons - remaining here seems to act as verb and gives awkward meaning.
E - 'it' is ambiguous can refer to civil war or slavery.

IMO B.

Though research remains to be done into the reasons why the Civil War was triggered, scholars do not regard slavery to be the sole cause.


A .Though research remains to be done into the reasons why the Civil War was triggered, scholars do not regard slavery to be the sole cause.
B. Though research remains to be done into what triggered the Civil War, scholars do not regard slavery as the sole cause.
C. Though the reasons that triggered the Civil War remain to be researched, slavery is not regarded by scholars to be the sole cause.
D. Despite research remaining into the reasons why the Civil War was triggered, scholars do not regard slavery as the sole cause.
E. Scholars do not regard slavery as the sole cause of the Civil War, though the reasons for it being triggered remain to be researched.

"The reason why" is not used in English academic writing, so A and D are out
The right idiom is "regard X as Y" not "regard X to be Y" , C out
E has "being" , hence B is correct!