Final answer:
The beach formation that extends from the mainland across a bay is known as a spit. This geological formation is created by the accumulation of sand or gravel due to coastal processes such as longshore drift, and option A: Spit, is the correct answer.
Step-by-step explanation:
When the beach extends from the mainland across a bay, a spit is formed. The correct answer to this question is option A: Spit. A spit is an accumulation of sand or gravel that projects from the land into a body of water, such as a bay or lagoon. This formation often occurs when longshore drift or other coastal processes carry sediment along the shoreline, and as the sediment reaches a point where the direction of the coast changes, it begins to accrue, forming a narrow extension into the water.
Examples of spits can be seen in various coastal regions around the world. These formations play critical roles in coastal ecosystems, acting as habitats for various species as well as natural protection barriers for shorelines against the forces of waves and tides. Landmass separations such as these sometimes hinder transport between different coastal habitats, according to the provided aerial photograph description.
It is also important to understand that spits are not to be confused with other coastal landforms such as atolls, which are coral islands encircling a lagoon, groynes, which are structures built perpendicular to the coast to prevent erosion, or deltas, which are often triangular-shaped landforms created at the mouth of a river where it slows down and deposits sediment.