Sand is supplied to the california coastal region primarily by:
• (a)coastal erosion •
• (b)local biological activity •
• (c)rivers that flow from the sierra-nevada mountains
•(d)ocean floor upwelling

I think the correct answer would be A. Sand is supplied to the California coastal region primarily by coastal erosion. It is a type of erosion that removes sand, sediments through the action of the waves, tidal currents or high winds in a certain beach system. The, the sands that had been eroded are moved to the coastal region of California. 

Ibelieve that a would be the best answer.