Be aware of preposition "
in" in the non-underlined part.
The uniformized set of characters, which some historians date in the late Qing dynasty, was the key to the sustainability and prosperity of the Chinese culture over thousands of years.
(A) The uniformized set of characters, which some historians date
Wrong. "
date in" is incorrect idiom. The correct one is "date at...."
(B) The uniformized set of characters, which some historians have thought to occur
Wrong. The "set of characters" cannot occur. (
logical problem)
(C) Uniformizing the set of characters, dated by some historians at
Wrong. Even though "date at" is correct idiom, C is still wrong because of "in" in the non-underlined ==> C will be: "dated by some historians
at in...". Hence, C is wrong.
(D) The uniformization of a set of characters, thought by some historians to have occured
Correct. (E) The set of characters' uniformization, dated by some historians to have been
Wrong. Same problem as in A. "
dated .....in" is wrong idiom.
Hope it helps.