According to the National Oceanic and Atmospheric Administration website, three types of plate boundaries can occur. These boundaries are known for how they "meet", such as the plates moving towards each other, the plates moving away from each other and finally the plates sliding past each other.
When the plates move away from each other, this is known as divergent boundaries. At divergent boundaries we find new land is formed from the rising magma to the surface.
When two plates slide past each other this is what is known as a transform plate boundary. What results is the land breaking off each other and causing tremendous stress. This breakage is what is known as a fault. Transform boundaries result in earthquakes.
Finally, comes the answer to your question. Convergent boundaries. Here boundaries collide together, with one boundary sliding beneath the other. This is known as subduction and is the cause for the formation of deep trenches.
So in regards to your question, the answer is B) crustal plates are moving towards each other.