It is helpful to know the definition of each option prior to making a selection.
A.) Transpiration is the evaporation of water through the leaves of plants.
B.) Groundwater is exactly that- water that flows underground! It is housed in soil pore spaces or fractures between rocks.
C.) Runoff (also known as overland flow) is the flow of water that occurs when excess stormwater, water that melts from mountaintops, or other sources flows to the surface of the earth.
D.) Evaporation occurs as water gains enough kinetic energy upon being heated and changes from a liquid to a vapor. Evaporation eventually condenses onto dust particles in the atmosphere to form clouds. When the condensed water droplets become heavy enough and fall to the Earth's surface, they are called precipitation.
Therefore, (A) and (D) are not likely answers since these processes are removing water from the surface of Earth. If it is not raining, runoff (C) is most likely not significant. Thus, the best answer would be (B), groundwater.