Which term describes an extended stretch of beach material that projects out to sea, connected to the mainland at one end?

The term that describes an extended stretch of beach material that projects out to sea, connected to the mainland at one end, is a "spit." A spit forms through the process of longshore drift, where sediment is gradually deposited in a coastal area, leading to a landform that extends into the water. This feature is often characterized by a curved shape at its outer end where the flow of water can create a hook, indicative of the sediment's movement.

In contrast, a beach is simply a shoreline area made of loose particles such as sand, gravel, or pebbles along a body of water but does not have the projecting characteristic of a spit. Coastal sand dunes refer to hills of sand formed by wind action along coastlines, typically found behind beaches and not extending into the sea. Rip rap consists of large, loose stones or broken concrete used to armor shorelines and prevent erosion and is not a natural beach feature that projects into the sea. Thus, the definition and characteristics of a spit align perfectly with the description provided in the question, making it the correct choice.