A "sink" is a term which describes a location which captures a particular substance. This question is asking for those locations which are considered areas where the largest amount of phosphorous is captured.
To identify the largest sinks for phosphorous, one must be familiar with the Phosphorous Cycle as well as the different chemical forms that phosphorous can take on. To varying extents, phosphorous moves through the biosphere, lithosphere, and hydrosphere. The biosphere is probably the most difficult to understand. It includes the sum of all ecosystems on a global scale. The rocky solid crust covering the Earth is treated as the lithosphere. The hydrosphere includes the total amount of water on the Earth. The Phosphorous Cycle includes the ways in which different chemical forms of phosphorous move within the biosphere, lithosphere, and hydrosphere to varying extents.
Although this is a complex process, it is possible to predict that the largest sinks for phosphorous include ocean sediments as well as terrestrial rocks. Such an assertion is based upon the fact that phosphorous is found in very large quantities within these areas as well as the total available global surface area associated with these areas.