Feature C is a stack, which is formed through a sequence of coastal erosion processes: 1. Waves erode a headland, often exploiting weaknesses like cracks, forming caves (like B). 2. These caves enlarge and may eventually meet through the headland, creating a natural arch (like D). 3. Continued wave erosion at the base of the arch and weathering on its roof weaken the structure. 4. Eventually, the roof of the arch collapses, leaving an isolated pillar of rock standing in the sea, which is the stack (C). That's 2 down. 3 left today — send the next one.