The correct answer is - b. increased dramatically.
In the early stages of the human development there hasn't been any changes in the amount of sediments in the rivers, but since the human kind started to settle in, perform activities that the Earth had never experienced before, and the sheer mass of the population, the sediments in the rivers have increased significantly.
The biggest impact the humans had had to contribute to this, is the erosion. Because of the needs the humans have, they have destroyed lots of natural cover of the soil, thus making it very easy to erode, and with the much increased erosion of the soil, the more sediments have started to go into the rivers.
