Final answer:
The gap between two plates forms and expands as they separate along a divergent boundary.
Step-by-step explanation:
The statement that best describes what happens between two plates as they separate is that the gap between the two plates forms and expands. This process is known as plate tectonics and occurs along divergent boundaries, where two plates move away from each other. As the plates separate, magma from the Earth's mantle rises to fill the gap, creating new crust. This can lead to the formation of new oceanic crust and the creation of features such as mid-ocean ridges.
Learn more about Plate tectonics