Which of the following is a geologic factor that best describes movement through groundwater?

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Movement of groundwater is governed by the properties of the geologic material through which water moves. Porosity—the fraction of void space in soil or rock—tells us how much water can be stored and how readily water can occupy pathways in the subsurface. When pore spaces are connected, water can flow through the material under hydraulic pressure, so higher porosity often corresponds to greater capacity for movement, provided the pore network is interconnected enough. The other options don’t describe a geologic control on subsurface flow: evaporation is a surface process; Henry's Law Constant relates to gas-liquid equilibria rather than groundwater movement; vapor pressure describes a liquid’s tendency to evaporate, not how water moves underground.