Question

Where might a spring form?

between the saturated and unsaturated zones
at the bottom of a deep valley
on a mountaintop at the origin of a stream
on the surface of a fast-moving aquifer

Answer 1

A spring forms where the water table (the boundary between the saturated and unsaturated zones) intersects the land surface.

So the best answer is: between the saturated and unsaturated zones.

Examples: that commonly happens at the bottom of a deep valley or where a permeable aquifer outcrops on a hillside. It is less correct to say “on the surface of a fast-moving aquifer” (springs are discharge where the aquifer intersects the ground, not simply the flow speed) and a mountaintop spring is possible only if the water table or a confined aquifer heads reach the surface there.