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Historians believe that, for most countries involved in the conflict, World War I's root causes were varied and complex, though many still point to the assassination of Archduke Franz Ferdinand as the immediate precursor to war.
(A) Historians believe that, for most countries involved in the conflict, World War I's root causes were varied and complex
Believe - correctly used for historians
For most countries - correctly modifies "believe"
Were - correct as per "causes"
No errors in this choice
(B) Historians believe that variation and complexity were there for most countries involved in the conflict by the World War I's root causes
"involved in the conflict by the WWI's root causes" - changes the meaning of the sentence. Provides a meaning that the countries were there in the war because of these causes - wrong
(C) The historians' belief is, for most countries involved in the conflict, root causes were varied and complex for World War I
Cannot find any grammatical/meaning flaw in this. Option A is just simpler
(D) For most countries involved in the conflict, the historians' belief that World War I's root causes were varied and complex
Here belief - is not a verb.
were - verb for causes
belief - does not have a verb
(E) For most countries being involved in the conflict, historians believe that the root causes that World War I had were varied and complex
"Believe that root causes that WWI has ..." - can be expressed in much simpler form as expressed in option A
Correct - Option A