The answer is c. Deciduous trees lose their leaves to prevent nutrient and water loss.
Losing leaves is a trees' strategy to survive cold winters. It is known that trees primarily lose water through leaves. In the winter, there is no enough water to replace that lost water. If the trees were not to throw away their leaves, they will soon dry out. Also, before falling the leaves begin, reabsorb all nutrients from the leaves and store it for the cold winter. Thus, losing leaves is important to prevent both nutrient and water loss during the winter.