Assertion: osmosis is one sided movement solvent molecules
Reason: in osmosis the next movement of solvent particles from lower concentration towards higher concentration takes place through semi permeable membrane

Dear Student, 

Osmosis is the phenomenon when the two solutions having different osmotic concentration are separated by a semi-permeable membrane and the movement of solvent takes place from a region of low concentration of the solute to a region where there is high concentration of the solute or from a region where there is high concentration of solvent to a region where there is low concentration of solvent.
Therefore, the assertion is wrong but the reason is true.
Hence, the correct option is D.
Regards!