The answer is; D
This is due to the fact that temperatures of a solvent are directly proportional to the amount of solute they can dissolve. The waters at the equator get heated more than those of other latitudes due to the angle of sun rays exposure and also the duration. This means that these waters can dissolve more salts per unit volume. As the currents push the waters towards the poles, the waters become cooler and loose some of the dissolved salts by precipitation once the saturation point is reached.