The answer to your question would be:: the wind because it carries the sand until it runs into an obstacle
Sand dunes: These are small hills made up of lose sand.
During formation of dunes ( in deserts or sea shores) there is creeping of sand particles along one side and there is sliding of sand on other side resulting in formation of a dune.
Thus the agent causes sand dunes to form is the wind carries the sand until it runs into an obstacle
