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.