“But they also” implies that ethno historians do the same thing as ethnographers.

A. Points out that they are not similar – using ‘unlike’ but they are similar in certain ways
- correct
B. They are similar and they are also similar – awkward changes the meaning
C. They are similar but they are also similar - awkward
D. They are not similar and they are similar – awkward
D. They are not similar but they don’t do something – does not make sense

OA could be A.

I will go for B.
What is OA?