When do rivers form deltas?

Correct answer: When a river deposits silt near the sea

A delta forms near the mouth of a river when the river loses speed and deposits the sediment it has been carrying. The deposited silt, sand, and clay can gradually build land that often has branching distributary channels. Therefore, option A is correct: a river forms a delta when it deposits silt near the sea or another standing body of water. Deltas are depositional landforms, not features made by the river drying or flowing backward.

At the river mouth, the current may slow because the channel enters relatively still water. As carrying power decreases, heavier and then finer sediments settle. Repeated deposition expands the sedimentary area and may create a roughly triangular or fan-like shape. The other options do not describe this process: ice on mountains, complete drying, and reverse flow are unrelated. Thus the key connection is river mouth plus sediment deposition, leading to answer A.