Explanation
'Had been’ is past perfect tense and the sentence describes that Suu Kyi was heading the struggle since 1988. So option B is the most appropriate word for the given sentence.‘Has been’ is present continuous tense whereas the paragraph is in past perfect tense. Thus, option A is not the appropriate word for the given sentence. Option C is incorrect because ‘was’ is simple past tense whereas the given sentence is in past perfect tense. Option D is incorrect because ‘is become’ is present tense whereas the sentence described is in past perfect tense.
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