Where two plates separate, or diverge, the boundary is termed a divergent plate boundary. The plates at these spots are moving apart and magma rises to fill in the separation resulting in lava flows and volcanoes. This divergence is also referred to as sea floor spreading. Divergence can also cause earthquakes and if it occurs near the continental crust, a rift valley will be created through faulting.
Since all of these are formed at divergent plate boundaries, the correct answer is a) Mountain formation. A mountain formation will NOT be likely at a divergent plate boundary. Convergent plate boundaries are where two plates collide or converge. Plate convergence that results in young mountains is called "orogeny."
It may be useful to click on the links below to see actual images that may clarify this sometimes confusing topic.