How do transform boundaries primarily move?

Transform boundaries primarily move horizontally past each other. This occurs because at these boundaries, tectonic plates slide laterally in opposite directions. The movement is characterized by the friction that arises between the edges of the plates, which can lead to seismic activity as stress builds up over time. When this stress is suddenly released, it results in earthquakes.

This horizontal movement distinguishes transform boundaries from convergent boundaries, where plates move towards each other, and divergent boundaries, where they move away from one another. The unique motion at transform boundaries is critical in understanding the geological activity associated with them, especially the formation of features like fault lines. Vertical movement is not a primary characteristic of transform boundaries, making the correct choice focused on the horizontal dynamics between the tectonic plates.