What's porosity?
a. the ability of water to trickle down through soil until it reaches groundwater
b. the ease with which water is able to flow from one pore to another in rock
c. a measure of the amount of groundwater available in the water table
d. a measure of the amount of open space within unconsolidated sediment or rock

D. A measure of the amount of open space within unconsolidated sediment or rock

Explanation:

The porosity is a measure that tells us how much of a rock or sediment is actually an open space. This is a very important characteristic of the rocks and soils, as it determines how much, how easily, and to what depth a water can go inside the rocks or soils. Pretty much all rocks, and all of the soils underlie under this measurement, as most of them have at least very small open spaces in them, while some have much larger and more prominent spaces that are easily noticeable.