How far can river run-off extend from its source?

The correct answer highlights the ability of river run-off to affect water quality and salinity levels significantly as it flows from its source out into the ocean. River discharges can extend considerable distances offshore depending on various factors, including flow rate, tidal effects, and ocean currents.

In this case, the choice suggesting that river run-off can reach out to 200 miles with an upper depth of 30-50 feet accurately captures a well-established range seen in marine studies. Research indicates that significant freshwater influence can be detected well past the immediate coast, particularly in areas with larger river systems. This extent allows the freshwater to create a distinct layer above the denser seawater, affecting stratification and ecological dynamics in the coastal marine environment.

The other options propose shorter distances, which do not fully encompass the extensive reach that larger river systems can achieve under certain conditions, thus highlighting the chosen answer's validity based on scientific observations in marine and environmental studies.