For a), you need to choose the graph that has a portion at the beginning where the distance from the shore stays constant, keeping in line with the narrative.
This narrows it down to choices 2, 3, and 4.
Then, the distance from the shore will go to 0 at a constant rate. The only graph remaining that does so is 3, therefore 3 is the answer.
For b), we need to choose the one that has the amount of water in the pool staying at half full for some portion of time. The only graph that does this is the 4th one, therefore, 4 is the answer.