Which process involves the cementation and hardening of rock fragments at the bottom of a water column?

The process that involves the cementation and hardening of rock fragments at the bottom of a water column is best described as diagenesis. Diagenesis refers to the whole range of physical and chemical processes that occur after sediments are deposited and have buried, resulting in the transformation into sedimentary rock. This includes processes such as compaction (where the weight of overlying materials compresses the sediments), cementation (where minerals precipitate from water and fill the spaces between sediment grains), and other changes that happen at relatively low temperatures and pressures near the Earth's surface.

While compacting is a key part of diagenesis, it specifically refers to the process of reducing the volume of sediment as a result of pressure from overlying materials, not the complete process of hardening through cementation. Metamorphism refers to the alteration of existing rocks under high temperature and pressure, and subduction involves the movement of tectonic plates, not sedimentation or diagenesis. Therefore, diagenesis encompasses both the compaction and cementation processes that lead to the hardening of sediments into rock.