Answer:
(D) A country's productivity could increase without significantly increasing the value of its currency.
Step-by-step explanation:
"And although in the past a few countries have deliberately kept their currencies undervalued, that is now much harder to do in a world where capital moves more freely."
This last passage indicates that since the movement of capital is not restricted, then developing countries are not able to manipulate their currencies, therefore, if the movement of capital were restricted, then developing countries could manipulate their currencies.