The correct answer is D. Water softening.
Water softening is a process that removes calcium and magnesium ions from hard water. These ions can cause scale buildup in pipes and appliances, and they can also make soap less effective. Water softening can be done using a variety of methods, including ion exchange, reverse osmosis, and distillation.
Sedimentation is the process of particles settling out of a liquid. This can be used to remove suspended solids from water, but it will not remove dissolved minerals like calcium and magnesium.
Filtration is the process of passing water through a porous material to remove particles. This can be used to remove suspended solids and some dissolved minerals, but it is not as effective as water softening.
Flocculation is the process of adding a coagulant to water to cause suspended particles to clump together. This can be used to remove suspended solids, but it will not remove dissolved minerals like calcium and magnesium.