The correct answer is:
D. has been volunteering
Explanation:
1. The sentence "Bill volunteers at a soup kitchen for the past week" is in the present perfect continuous tense.
2. The use of "has been volunteering" in option D matches the tense of the original sentence.
3. "Has been volunteering" indicates that Bill started volunteering in the past and has continued to do so up to the present moment.
4. Therefore, option D is the best replacement for the underlined part of the sentence.
