Final answer:
A rift valley in the middle of an island is most likely caused by a divergent plate boundary.
Step-by-step explanation:
If a rift valley is found right in the middle of an island, it is most likely caused by a divergent plate boundary. A divergent plate boundary occurs when two plates move away from each other, creating a gap where magma rises to the surface, forming a rift valley. The best-known example of this is the East African Rift System, which stretches through several countries in eastern Africa.
Learn more about Plate Tectonics