The correct answer is: D. Jack said that he had finished it several days earlier.
The sentence “I finished it several days ago” is in the past perfect tense. The past perfect tense is used to talk about an action that happened before another action in the past. In this case, the action that happened before is “Jack said”. So, the sentence means that Jack finished the task several days before he said it.
The other options are incorrect because they do not use the past perfect tense. Option A uses the past tense, which would mean that Jack finished the task at the same time as he said it. Option B uses the present perfect tense, which would mean that Jack has finished the task at some point in the past and it is still finished now. Option C uses the simple past tense, which would mean that Jack finished the task at some point in the past but it is not necessarily finished now.