Based on the scenario described, where two tectonic plates are moving apart, the most appropriate diagram would be:
b
Diagram B: A new plate from underneath filled in the gap that was created by the plates moving apart.
This is because when tectonic plates move apart, such as at a divergent boundary, magma rises from the mantle to fill the gap, creating new crust. This process often forms new oceanic crust if the plates are oceanic, or new landforms if they are continental.