In the last few thousand years, sea-level changes have resulted from all but in the last few thousand years, sea-level changes have resulted from all but the change of the total volume of water in the earth system deglaciation submergence of land masses emergence of land masses eustatic changes

The correct answer is - the change of the total volume of water in the Earth's system.
The total volume of the water in the Earth's system is pretty much the same, and for it to change, and change the levels of the water in the oceans and seas, the Earth will have to go over a totally new conditions where either a new influx of water will reach our planet, or the heat will increase and the water will evaporate, for now that is not a possibility. 

It is c. naturally in the environment