"Regarded to be" is incorrect and "reasons why/reasons being" is also incorrect.
(A) Though research remains to be done into the reasons why the Civil War was triggered, scholars do notregard slaveryto be the solecause.
(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 notregarded by scholarsto be the sole cause.
(D)
...
(A) Though research remains to be done into the reasons why the Civil War was triggered, scholars do notregard slaveryto be the solecause.
(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 notregarded by scholarsto be the sole cause.
(D)
...
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