Question

what causes the movement of tectonic plates?
erosion and sedimentation
gravity alone
convection currents in the mantle
volcanic activity

Explanation:

The Earth's mantle has convection currents. These currents are due to heat from the core. The movement of the mantle material causes the tectonic plates (which are like 'floats' on the mantle) to move. Erosion and sedimentation affect the Earth's surface but not plate movement. Gravity alone isn't the main cause. Volcanic activity is more of a result (in some cases) of plate movement rather than the cause.

Answer:

Convection currents in the mantle