Which zone is characterized by having all pore spaces filled with water?

The saturated zone is characterized by all pore spaces being filled with water. In this zone, also known as the phreatic zone, groundwater is present in the soil and rock pores, creating a situation where the water pressure is equal to or greater than atmospheric pressure. This environment is crucial for aquifers because it is the source of groundwater that can be extracted through wells.

In contrast, the unsaturated zone, which lies above the saturated zone, contains both air and water in its pore spaces. The water table zone is a transitional area where the pore spaces are partially filled with water, intersecting the two zones. The term "aquifer zone" generally refers to a geological formation that can store and transmit water, but this term does not specifically indicate the condition of the pore spaces like the saturated zone does. Thus, the saturated zone clearly defines the condition where all pore spaces are filled with water, making it the correct choice.