I believe D because the sentence states,"During my visit to the shelter I learned that it paid TWO STAFF MEMEBERS and 13 volunteers spend time with every animal. So what it sounds like is animal lovers work there you have to love a the animals in order to do your work, not only this but the answer D states there are plenty of workers to give the animal good care! It wouldn't make sence if it was any other answer. So thats why I think D is the best choice for you!
From the definition of absolute phrases we know that it is a modifier that attaches to a sentence without a conjunctution. So, we can say "his chubby legs, pumping" is the correct answer.
