The correct answer is - a. flow.
The volume of the water that is moving past a certain point of the river, in a precisely given time-frame is the river-flow, or also referred to as stream-flow.
The river-flow is not the same at all points of the rivers, or in all parts of the year. The lower part of the river has a bigger flow because the river is bigger, thus has more water, and vice versa, the upper part is smaller, has smaller amount of water, thus has smaller flow. Also, the different periods of the year determine the river-flow. When there's bigger amount of rainfall, or snow melting, the river gets more water, and the flow is bigger, but if there's a drought for a prolonged period of time, the flow is getting smaller.
