Final answer:
Option A.
The speed at which a river flows is affected by the slope of the riverbed, the volume of water present, and obstacles within its course. Consequently, option a) is the correct answer.
Step-by-step explanation:
The factors that affect how fast a river flows include a) The slope of the riverbed, water volume, and obstacles in the river.
The slope of the riverbed influences the gravitational force acting on the water, with steeper slopes causing water to flow faster.
Water volume, referring to the amount of water in the river, affects flow rate because higher volumes typically lead to swifter currents.
Obstacles in the river, such as rocks, fallen trees, or even man-made structures, can impede water flow and affect its speed.
Therefore, the correct option in the final answer regarding what three factors affect how fast a river flows is option a).