Which of the following favours the formation of rift valleys? I. Tensional forces in the earth's crust II. Compressional forces in the earth's crust III. La...

The correct answer is option A: I and III only. Rift valleys are formed due to tensional forces in the Earth's crust which pull the crust apart, causing it to stretch and thin. This leads to the formation of a depression or a valley, which is further deepened by erosion and other geological processes. Lateral forces and compressional forces, on the other hand, result in the formation of folds, mountains, and other geological features. The subsidence of the rift valley floor occurs after the valley has been formed, as a result of the continued movement of the Earth's crust.