Which geological feature forms when two tectonic plates move apart?

The rift valley is formed when two tectonic plates move apart, a process known as divergence. As the plates separate, the land between them sinks down, creating a valley. This formation is often observed in continental rift zones, where the Earth's crust is being pulled apart.

In contrast to other geological features: oceanic trenches are typically formed at convergent boundaries where one plate subducts under another; subduction zones are associated with the same convergent processes; and fault lines are fractures within the Earth's crust where movement has occurred but do not specifically denote the separation of plates. Therefore, the formation of a rift valley specifically indicates the divergent movement of tectonic plates, making it the correct choice.