Final answer:
If a spit grows and isolates a lagoon from the sea, it becomes a Barrier Island, a landform parallel to the shore that serves as a protective buffer and ecological habitat.
Step-by-step explanation:
If a spit continues to grow and eventually cuts off a lagoon from the sea, it can form a Barrier Island. A barrier island is a coastal landform that acts as a barrier between the ocean and the mainland, and is typically parallel to the coast. These landforms are significant as they protect coastlines from severe storm waves and wind. They also provide habitat for various wildlife and support a unique ecological environment.
In this specific context, you mention a spit that could close off a bay or lagoon, which aligns with the formation of a barrier island. A tombolo is a type of sandbar that connects an island to the mainland, which is not the case here. A tidal flat is an area that is exposed at low tide and submerged at high tide, typically not forming a barrier. A sea arch is a natural rock formation where an arch has formed with an opening underneath, which is also different from the situation described.