What is the term for a flat, broad beach?

The correct term for a flat, broad beach is commonly referred to as an emergent shoreline. This term describes a coastal area that is experiencing tectonic uplift or a decrease in sea level, leading to a wider and flatter beach formation. Emergent shorelines often feature sandy beaches that are expansive and can be found in regions where geological processes allow for such features to develop.

Emergent shorelines are characterized by their broad nature, which creates ample space for recreational activities and habitats for various coastal organisms. The flat topography is a result of the sedimentary processes and the historical geological activity in the area.

In contrast, the other terms do not accurately describe the features of a flat, broad beach. For instance, a neutral shoreline doesn't denote any specific geological characteristic. Steep shorelines refer to cliffs or abrupt changes in elevation from land to sea, while submerged shorelines indicate areas that are underwater and hence not flat beaches as would be expected in the context of a broad sandy area.