The correct answer is option 4: Fiords. Stalactites, stalagmites, and sinkholes are all examples of geological formations that are formed as a result of underground water action. Stalactites are icicle-like formations that hang from the ceiling of caves, while stalagmites are formations that grow from the floor of caves. These formations are created when water containing dissolved minerals drips from the ceiling and deposits the minerals over time. Sinkholes, on the other hand, are depressions or collapses in the ground that occur when underground water dissolves soluble rocks, such as limestone, creating voids or cavities that eventually collapse.

Fiords, however, are not formed by underground water action. Fiords are long, narrow inlets with steep sides or cliffs, usually created by glacial activity. Glaciers carve out deep valleys which later become flooded by rising sea levels, forming fiords. Therefore, the correct answer is option 4.