IMO D

'After' along with 'since' seems awkward in A,B abd E

also B uses were the former East Germans which is wrong as we are talkin bout 1 grp. Had been is not required in A and E

As since 1989 is used, we need continuation hence 'has been' is better than 'was' so IMO C is not correct.

IMO "D".

Since 1989, after the Berlin Wall had been demolished, one of the most problematic ethnic groups in the reunified Germany, in cultural and economic assimilation terms, was the former East Germans, who have had to acclimate to an entirely different political system.

a) after the Berlin Wall had been demolished, one of the most problematic ethnic groups in the reunified German, in cultural and economic assimilation terms, was the former East Germans
IMO Since + after togehter is redundant....Also since the logic of sentence requires that the condition continues in the present...so we need to have present tense...which is the case with "D"....

b) after the Berlin Wall was demolished, one of the most problematic ethnic groups in the reunified Germany, in cultural terms as well as those of economic assimilation, were the former East Germans
Breaks One of the Xs <singular> paradigm.

c) when the Berlin Wall was demolished, one of the reunified Germany's most problematic ethnic groups, in terms of cultural and economic assimilation, was the former East Germans
Because non underline part contains "since", so we need perfect tense...like has been

d) when the Berlin Wall was demolished, one of the most problematic ethnic groups in the reunified Germany, in terms of cultural and economic assimilation, has been the former East Germans

e) after the Berlin Wall had been demolished, one of the most problematic ethnic groups in the reunified Germany, in both terms of cultural and economic assimilation, have been the former East Germans
Breaks One of the Xs <singular> paradigm.

damn....I wish I an eye for such errors....damn....

GMAT in a week....damn ....damn...

btw, well done guys...OA is what you said it is

The subject of the sentence is "one of the most problematic ethnic groups", a singular noun.
The verb, however, is "were," which is plural. We need to find a choice that uses a singular
verb instead.

(A) This choice is the same as the original sentence.

(B) This choice does not correct the subject-verb issue; it still uses "were" to refer to "group."

(C) This choice uses the singular "was," but the simple past is not the appropriate tense here because
of the ongoing nature of the problem. Moreover, "the reunified Germany's most problematic ethnic
groups" is an awkward construction.

(D) CORRECT. This choice uses the singular "has been," which is also in the present perfect tense,
indicating the ongoing nature of the problem. Moreover, "in terms of cultural and economic
assimilation" is a more idiomatic and elegant phrasing than that of the original.

(E) This choice does not correct the subject-verb issue; it uses "have been" to refer to "group."