The most reasonable answer i think would be option b: building pedestrian-friendly walkways.
Why? because if they would have to purchase bikes for each household, that would be too expensive.
I think option 3 does not even go in these categories.
And option 4: think not all senior citizens go to the gym. They usually might just walk around the neighborhood once or twice, and go to bed, because their legs hurt.
That is why i think option 2 fits best here.
Hope i helped!
