The correct option is D had she seen
Let’s examine the sentence: “Rarely ____ such terrible poverty as in this African city.” All the options talk about her having seen the poverty. As she had already seen the poverty in the African city, the tense becomes past perfect. Past perfect tense indicates that an action was completed (finished or "perfected") at some point in the past before something else happened. This tense is formed with the past tense form of "to have," i.e. had, plus the past participle of the verb. Only options A and C meet these conditions. Of these two options, Option C is more appropriate as using ‘had’ before ‘seen’ puts an extra emphasis on the fact stated in the sentence. Hence, Option C is the correct answer.