OA: A

reasoning:

we are talking about past first and then present

b)thinking...continuous tense is wrong, we need past tense for the fist part "once thought"

c) we don't need HAD here because order of events is clear with simple past (thought) and then present tense (understand)

d) subject of the sentence is scholars NOT the Native American literatures


Scholars who once thought.......................... now understand this

scholars can understand not the literatures.

e) this is completely non-sense

scholars once, thinking


usage of ,thinking is wrong as here there isn't any cause-effect relation or the -ing form id not needed to modify the preceding clause.


Hence, correct answer: A