One of the three proposed theories of barrier island formation is:

• A. Waves transport sand landward until a sandbar is formed, and it grows into a barrier island
• B. Elongate sand spits connected to the mainland become isolated in violent storms
• C. Dunes in low-lying coastlines become barrier islands with rising sea level
• D. Tidal flats convert into barrier islands due to sea level rise

Answer: (A)

The main idea is that sediment is moved along the shore by waves and longshore currents, building a sandbar parallel to the coast. As sand accumulates landward and gains height, this continuous supply creates a distinct, raised landform that separates from the mainland, becoming a barrier island with a lagoon or inlet behind it. This wave-driven, landward growth of a sandbar into a barrier island is the best-supported mechanism because it directly explains how extended sand accumulations can evolve into a coastal island set behind a protective, shallow water body.

Other scenarios describe plausible local outcomes, but they don’t capture the typical, process-driven path. For example, storms might cut or cut off connections in some cases, or sea-level rise can interact with dunes or tidal flats, but the classic wave-built sandbar–to–barrier-island sequence remains the central concept.