The correct answer is D. walked.
The sentence “When she was younger, she walked five kilometers a day” is in the simple past tense. The simple past tense is used to talk about things that happened in the past. In this case, the speaker is talking about something that the woman did when she was younger.
The other options are not correct because they are not in the simple past tense. Option A, had been walking, is in the past perfect progressive tense. The past perfect progressive tense is used to talk about things that happened before another event in the past. In this case, the speaker is not talking about something that happened before another event in the past. Option B, has been walking, is in the present perfect progressive tense. The present perfect progressive tense is used to talk about things that started in the past and continue to the present. In this case, the speaker is not talking about something that started in the past and continues to the present. Option C, had walked, is in the past perfect tense. The past perfect tense is used to talk about things that happened before another event in the past. In this case, the speaker is not talking about something that happened before another event in the past.