Which process involves the sinking of a coral island below sea level to form a seamount?

The process that involves the sinking of a coral island below sea level to form a seamount is subsidence. Subsidence refers to the gradual sinking or settling of the Earth's surface due to various factors, such as geological activity or the compaction of sediments. In the case of coral islands, they can undergo subsidence as a result of the geological processes that cause the underlying oceanic crust to sink. Over time, as the coral island sinks, it can eventually be submerged, resulting in the formation of a seamount.

This is distinct from the other options. Erosion refers to the wearing away of land or rock, which does not lead to the sinking of the island itself. Subduction is a tectonic process involving one plate moving under another, which can lead to volcanic activity and the creation of deep ocean trenches rather than the simple sinking of coral islands. Sea-level rise is a phenomenon that occurs when water levels increase due to melting ice caps or thermal expansion but does not directly lead to the sinking of the island in the geological sense like subsidence does. Thus, subsidence is the correct term that describes the process involved in the sinking of a coral island to form a seamount.