Experts please clear one thing although Regard as is the correct idiom but In option B don't you thing there is a modifier error. The first independent clause in B talks about reason so second clause must start with the reason (
Slavery ).
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.