Answer: A. when the water current slows down.
Step-by-step explanation:
When a water current slows down, the force of the water is not strong enough to continue carrying the sediment, so it drops the sediment and deposit it. The slower the flow, the larger the particles that can be carried and eventually deposited. This is why river deltas, for example, are formed where a river enters a body of water and the flow slows down, allowing sediment to build up and create landforms.