The correct answer is A. solifluction.
Solifluction is a type of mass movement that occurs in periglacial environments, which are areas where the ground is frozen for at least part of the year. It is caused by the thawing and freezing of water in the soil, which causes the soil to move slowly downslope. Solifluction is a significant process in shaping the landscape in periglacial environments.
B. Gelifluction is a type of solifluction that occurs in areas with permafrost, which is ground that is frozen year-round. Gelifluction is caused by the thawing of the permafrost, which causes the soil to become saturated with water and move slowly downslope.
C. Frost creep is a type of solifluction that occurs in areas with seasonal frost, which is ground that freezes during the winter and thaws during the summer. Frost creep is caused by the repeated freezing and thawing of the soil, which causes the soil to move slowly downslope.
D. Rockfall is a type of mass movement that occurs when rocks break loose from a cliff or other steep slope and fall to the ground below. Rockfall is caused by a variety of factors, including weathering, erosion, and earthquakes.