Final answer:
The speed of the boat in still water is equal to the speed of the current of the river.
Step-by-step explanation:
To determine the speed of the boat in still water, we can use the equation:
speed of boat in still water = speed of current + speed of boat relative to the ground
Since the boat travels upstream and downstream, the speed of the current and the speed of the boat relative to the ground cancel each other out in each direction. Therefore, the speed of the boat in still water is equal to the speed of the current.
In this case, the current of the river is 2 mph, so the speed of the boat in still water is also 2 mph. Therefore, the answer is option a. 4 mph.