Which characteristic best distinguishes runoff and infiltration?
a. runoff involves condensation and infiltration involves evaporation.
b. runoff occurs over land and infiltration occurs underground.
c. runoff occurs during floods and infiltration occurs during drought.
d. runoff describes water movement and infiltration describes water storage.

The right option is; d. Runoff describes water movement and infiltration describes water storage.

Runoff is the portion of precipitation, irrigation or snow melt that flows over land as surface water instead of infiltrating or evaporating. Factors that influence runoff are permeability, amount of rainfall, slope and vegetation.  

Infiltration is the process by which water or precipitation on the ground surface moves into the soil. Factors that determine the rate of infiltration include; soil type, topography of the land, amount of precipitation, vegetation, and pre-saturation levels.

b. Runoff occurs over land and infiltration occurs underground.

Explanation:

Runoff and infilteration both are the phenomena which effect the fertility of the soil. A runoff can be defined as the flow of water on the surface of the earth. The water of runoff may come from rainwater, melting of the glacier ice, and irrigation. It removes the topsoil and subsoil layers. This phenomena effects the fertility of the soil because of the loss of surface minerals.The infilteration can be defined as the process of downward flow of water inside the soil. This leads to the leaching or removal of the inorganic and organic minerals from subsurface and deep layer of the soil.  

On the basis of the above explanation, b. Runoff occurs over land and infiltration occurs underground. is the correct option.