Choose the phrase that best describes work being done by someone's arms.
a. a child building a sand castle
b. a mother holding a baby
c. a boy pushing on a tree
d. a traveler carrying luggage

The correct answer is b. a mother holding a baby. 

Babies could be fragile. Thus, it will require an effort when one is holding a baby in their arms. This is because they should be strong and firm at the same time they should be soft and comfortable for the baby. Also, a mother's touch is different. So the best work done in arms is b. a mother holding a baby. I hope that this answer helped you.