Answer: C. They are formed due to the action of waves.
Step-by-step explanation:
A stack is a column (usually of rock) that protrudes in the sea or near the coast by the action of the erosion of waves when they hit the rock formation. On the other hand, a spit is an accumulation of sand that sometimes rises above the surface, which forms along a shoreline or near a river or lake, due to the currents formed by the waters.
Therefore, both geological formations have in common the fact that they are caused by the action of the waves.