Solifluction is a type of geomorphic process that occurs in permafrost regions, making option 3 the correct answer. The process involves the slow flow of water-saturated soil down a gentle slope. During the warmer periods, the upper-most layer of the permafrost melts and causes the soil above to become waterlogged and move slowly downhill due to gravity.

Option 1, desert regions, is incorrect because solifluction requires water, which is scarce in a desert environment.

Option 2, karst regions, is also incorrect. Karst regions are characterized by soluble rock such as limestone, which results in distinctive topography characterized by sinkholes and caves, not soil flows.

Option 4, savanna regions, is incorrect as well. Savannas are marked by grasslands and scattered trees, and while they can experience heavy seasonal rains, they are not characterized by permafrost and hence do not undergo solifluction.

Hence, the unique permafrost environment that permits solifluction to take place is found in permafrost regions, not in desert, karst, or savanna regions.