The correct answer is option 4: Karst. A sinkhole is a natural phenomenon that occurs in areas with karst topography. Karst topography is characterized by soluble rocks, such as limestone, dolomite, or gypsum, that can be dissolved by water. Over time, this dissolution creates underground channels and cavities. When the roof of one of these underground cavities collapses, a sinkhole is formed on the surface. Sinkholes can vary in size, from small depressions to large craters.

Option 1: Plain is not correct because sinkholes do not typically form in flat, low-lying areas like plains.

Option 2: Desert is not correct because sinkholes can occur in any type of environment, not just deserts.

Option 3: Tundra is not correct because sinkholes are not commonly found in cold, frozen environments like the tundra.

Therefore, the correct answer is option 4: Karst.