How did the Basaltis Vallis on Venus form?

Basaltis Vallis on Venus formed primarily by the processes related to volcanic activity, specifically through the movement of lava. When considering the option that lava drained from beneath the crust, it's essential to understand how volcanic formations occur. In many volcanic environments, including Venus, magma from the mantle rises to the surface, creating large flows of basaltic lava that can carve out valleys as they spread and solidify. This phenomenon reflects a volcanic origin rather than tectonic movement or erosion.

The distinction between volcanic formation and other geological processes is crucial. While tectonic movement can create rifts and faults, it does not directly lead to the formation of lava flows, which is vital for creating features like Basaltis Vallis. Similarly, erosion involves the breakdown and removal of surface materials by environmental factors, not the active formation of volcanic landscapes. Thus, the notion that lava drained from beneath the crust accurately captures how the Basaltis Vallis was created, aligning with our understanding of Venus' geological processes and its volcanic history.