Select the correct option: Only after ten years of fieldwork __ she truly understand the dialect’s nuances. 

The correct option is (b). When a sentence begins with a negative or limiting adverbial phrase such as  Only after, Only then ,  Never, Hardly , etc., we use  inversion . Inversion means the auxiliary verb comes before the subject, similar to a question form.  Here, the main verb is  understand  in the simple past context (because the action of understanding happened after ten years). So we require the auxiliary  did  for inversion:  Only after ten years of fieldwork did she truly understand... Grammatical rule used:  Limiting adverbial at the beginning +  auxiliary + subject + main verb  (inversion). Example (rule):  Only then  did  he realize his mistake.  Why other options are incorrect: Option A does not form correct inversion and also mismatches tense logic. Option C is not suitable because  was  is not the correct auxiliary for the verb  understand . Option D keeps normal order (no inversion), which is incorrect after  Only after  at the start. So the correct answer is (b)