Final answer:
A delta is formed when a slow river deposits sediment at its mouth near the ocean, where the water slows down considerably. High silt content and minimal ocean currents contribute to the formation of deltas, which are often triangular-shaped regions of lowland comprised of sand, silt, and clay.
Step-by-step explanation:
The formation of a delta is best described by the process where a slow river deposits sediment at its mouth near the ocean. This occurs when a river reaches a large body of water, like an ocean or a lake, and the water flow slows down significantly. The sediments, which include sand, silt, and clay, that have been carried downriver start to settle. This happens especially in areas where the ocean has minimal currents and wave action, leading to the accumulation of these materials and the formation of low-elevation areas known as deltas, which are often triangular in shape.
Rivers with high silt content are particularly good at creating deltas, whereas rivers with low silt content or those that empty into areas of high wave action or currents tend to form estuaries. The depositional environment can vary, and in the process of delta formation, the deposited sediment can contain different materials such as gravel, sand, or clay depending on the energy of the water.
Thus, the correct answer to which best describes how a delta is formed is: 3) A slow river deposits sediment at its mouth near the ocean.