When waves of water approach the beach, they do so as an angle because they have been slowed down by the first parts of the beach that they have encountered.
When they hit the beach at an angle, they come back to the water and keep doing so like the movement shown in option D. This constant motion moves the sand and other particles on the beach progressively and the current that does so is the Longshore current which <em>moves parallel</em> to the coastline.
