Osmosis occurs spontaneously in response to a driving force. The driving force here refers to what?

 Osmosis is the process of diffusion of  solvent from solvent compartment to solution comparment having the same solvent or from dilute solution to concentrated solution, which happens through a semi permeable membrane (SPM). A semipermiable membrane can be natural one like egg membrane, plant cell walls etc or a synthetic one like copper ferrocyanide depositied in the tiny pores of a porous wall.

Now what drives this?  As we know vapor pressure is high over solvent and low above solution. Again it is high above dilute solution and low above concetrated solution. This vapour pressure difference is the actual driving force for osmosis to take place

One may wonder the vapour pressure of a solution is never going to equal that of a solvent at a given temperature, what ever be the extent of dilution, so will osmosis continue until the wholew solvent is dried? never. there is another factor that operates. Due to osmosis the height of the solution column would rise up and when it rise above the solvent column, an excess mechanical pressure develops on that side. Then osmosis stops wen vapor pressure of solvent = vapor pressure of solution + mechanical pressure of the solution column above the common level(this is found out using p =h xd x g)

Regards :D